BC Transit â Air Brake Course

BC Transit – Air Brake Course 

Presented By: Safety, Security & Training Department 

Revised December 2010 

1


Learning Objectives: 

1. Air Brakes Introduction 

2. Definitions & Concepts 

3. Five Main Components 

4. Basic Air Brake System 

5. Other Air Brake System Components 

6. Single Circuit 

7. Dual Circuit 

8. Trailers 

9. Off‐Highway Units 

10. Summary, Questions, Wrap‐Up 

2


1. Air Brakes Introduction: 

a) Resources 

b) Homework 

c) Quizzes 

d) ICBC Air Brake Theory Exam 

e) Air Brake Endorsement 

3



a) Resources 

• ICBC Driving Commercial Vehicles Guide 

(Chapters 1, 2, 8, 9 & 10 –see page 2) 

• Richmond Public Library Practice Test 

http://www.yourlibrary.ca/driving/ 

• BC Transit Website 

(Being developed –see BC Transit –Air Brake Course Handout in 

Interim) 

4



b) Homework 

• Day 1: Chapter 1 & 2 (Read & Questions), plus Chapter 8 (Read) 

• Day 2: Chapter 8 (Questions), Chapter 9 & 10 (Read & Questions) 

c) Quizzes 

• Day 1: Air Brake Course ‐ Chapters 1 & 2 –Review 

• Day 2: Air Brake Course –Final Exam 

5



d) ICBC Air Brake Theory Exam 

• Cost is $15 

• Letter of Verification required (bring this to ICBC on exam day) 

• Multiple Choice Exam technique (read each question twice thoroughly, 

only answer if completely clear –otherwise skip question) 

e) Air Brake Endorsement 

• After successfully completing the ICBC Air Brake Theory Exam, a 

practical evaluation will need to be completed (this is performed during 

the ICBC Road Test & Pre‐Trip –must be completed within 1 year of 

completion of Theory Exam, or Theory Exam would have to be redone) 

6


2. Definitions & Concepts: 

a) Traction, Heat, Friction 

b) Weight & Speed – Stopping Distance 

c) Force Multiplication 

d) Compressed Air 

e) Combining Compressed Air with Force Multiplication 

7



a) Traction, Heat, Friction 

8

The final factor that will determine 

if a vehicle will move is traction. 

The final factor that will determine if 

a vehicle will stop is traction. 

Traction: the ability of a tire to grip the road surface over which it rolls. 

Friction: force that resists movement between two surfaces in contact with 

each other. 

The engine of this truck converts the energy of heat 

into the energy of motion. The brakes of this truck 

must convert the energy of motion into the energy 

of heat. The friction between the the brake linings 

and drums generate heat while reducing the 

mechanical energy of the revolving drums and 

wheels. This energy is absorbed by the drums and 

dissipated as heat to the atmosphere. 

9



b) Weight & Speed – Stopping Distance 

10

Weight - Speed - Distance 

When weight is doubled, the braking power to stop in the same 

distance, must double! 

When speed is doubled, the braking power required to 

stop in the same distance must increase 4 times! 

When weight and speed are doubled, the braking power 

to stop in the same distance must increase 8 times! 

A vehicle carrying a load of 14,000 kgs at 16 km/h is brought to a stop in 30 meters 

under normal braking. If this vehicle’s weight increased to 28,000 kgs., and the 

speed was increased to 32 km/h, it would require 8 times the braking power to stop 

in the same 30 meter distance. Remember, this is under ideal conditions. If traction 

were reduced by poor road conditions, what effect would this have on overall 

stopping distance? 

11

Stopping Distance 

Stopping distance consists of four factors... 

1) Perception Time: (also described as see-think time) = the time it 

takes the brain to recognize a hazard. Usually 3/4 second or 13 meters travel. 

2) Reaction time: (also described as Do Time) = it is the time required 

for your brain to tell your foot to apply the brakes. Usually 3/4 of a second 

or 13 meters travelled. 

3) Brake Lag: the time required for air to travel through a properly 

maintained air brake system and actually begin applying the brakes. 

Usually 4/10 of a second. 

4) Braking Distance: the time or distance a vehicle travels before 

it stops after the brakes have been applied. 

12

Perception Time (See-Think) 

Perception Time, generally, is 3/4 

second. This is the time it takes for us to 

perceive that there might be a need to 

stop or take some other defensive action. 

At 50 km/h, your vehicle will travel 13 

meters (or one bus length) before your 

brain even decides it would be a good 

idea to do something about the hazard. 

What types of techniques can you think of 

that would help us reduce or compensate 

for this 3/4 second delay? 

13

Reaction Time (Do) 

Reaction time, also generally about 3/4 

second. This is the time it takes for us to 

react to a need to stop or take some other 

defensive action. Again, at 50 km/h, 

your vehicle will travel 13 meters (or 

one bus length) over the road before 

the brakes even begin to stop the 

vehicle. Here is where defensive driving 

techniques can make for a whole lot less 

accidents. 

14

4/10 sec. 

Brake Lag 

(mechanical lag) 

Brake lag is usually 4/10 

second, in a properly 

maintained system. It 

doesn’t sound like much, but 

consider once again at 50 

km/h your vehicle will 

travel nearly 10 meters 

before the brake linings 

come into contact with the 

drums. 

15

Braking Distance 

This distance is dependent on many 

factors. The condition of the brake 

components, brake adjustment, 

road, operator, speed and load of the 

vehicle will all determine how much 

asphalt disappears under your bumper 

before coming to a stop.The 

professional driver realizes the 

limitations of his/her vehicle’s 

brakes and adjusts their driving 

accordingly. 

16

Stopping Distance…(total stopping time) 

•is the total distance travelled before your vehicle stops after factoring in 

perception time, reaction time, brake lag time and braking distance. 

• Other factors such as poor road condition due to inclement weather, driver 

fatigue and vehicle condition will add length to overall stopping distance. 

•As a professional driver, it is incumbent upon you to prepare yourself for any 

eventuality and govern your driving habits accordingly. 

•Techniques such as brake coverage, eye lead time, effective scanning and 

comprehensive vehicle pre-trip inspections can do a lot to even the odds. 

• Your passengers, other road users and your own safety depend on your attitude. 

Think professional, act professional and professional you will be! b 

17

RoadSense Tip 

•Total Stopping Time will be even longer if brakes are incorrectly y adjust or 

vehicle is on a downgrade. 

•The most common air brake system defect found during a commercial l vehicle 

inspection is brakes that are out of adjustment. 

18



c) Force Multiplication 

19

Force Multiplication (Mechanical Advantage) 

100 

4 1 

A 

C 

B 

400 

“…give me a lever long enough and a place to rest it, and I will lift the world…” 

Braking systems use devices to gain a mechanical advantage. The 

most common device for this purpose is leverage. In this simple 

example, a lever is placed on a pivot called a fulcrum. The distance 

from A to C is 4 feet, and the distance from C to B is 1 foot. Therefore 

the ratio is 4:1. If a 100 lb. downward force is exerted against point A, 

an upward force of 400 lbs. is achieved at point B. 

20

Apply the principal from the previous slide to the example below: 

C 

A 

D 

B 

“A” represents the slack adjuster in 

the foundation brakes. As pressure is 

applied, the slack adjuster works as a 

lever, on shaft “D.” As the “S”cam 

attached to the shaft rotates, a 

mechanical advantage is achieved at 

points “B” and “C”. If the slack 

adjuster were 6 inches in length, and 

the “S” cams were 1 inch in length, 

the ratio would be 6:1. 

21



d) Compressed Air 

22

Using Compressed Air to Our Advantage... 

Atmospheric Pressure 

15 lbs./sq. in. 

Suppose the walls of the room you are sitting in now, were to begin to squeeze 

together evenly in all directions. Assume the room is sealed and no air can escape. 

• What would happen to the atmospheric pressure as the size of the room began to 

shrink? 

•Would the pressure in the room begin to increase? 

•Would the rate of increase be proportional to the reduction in the size of the room? 

•What would the pressure be like in the “red” room? 

23

Compressed air is air that has been forced into a space 

smaller than it would normally occupy. 

Rather than squeezing the room smaller, suppose we instead forced air into a 

chamber or container... 

As the compressor pumps air (forces air into the 

cylinder), the pressure within the cylinder 

(reservoir) begins to increase. That pressure is 

exerted against all the surfaces of the reservoir at 

the same time, and at the same pressure.This 

compressed air can then be used to mechanical 

advantage in the operation of the vehicles brakes. 

24



e) Combining Compressed Air with Force Multiplication 

25

Obtaining the mechanical advantage... 

The reservoir below, contains compressed air at 10 pounds per square 

inch (psi). (atmospheric pressure is considered to be 0.) 

0 5 10 25 30 35 40 

1 inch dia. 

made in 

Canada 

pounds per sq. in. 

If a constant supply of compressed air were directed through a pipe one inch 

in diameter, and a one inch plug were placed in the pipe, the compressed air 

would push against the plug. Holding a scale against the plug would register 

how many pounds of force were being exerted against the plug. 

26

Combining mechanical advantage to achieve force multiplication. 

Air 

pressure 

at 100 psi. 

3000 lbs 

of force 

Slack adjuster - 6 ins. in length 

100 psi 

Brake chamber - 30 sq. in. diameter 

18,000 ft.-lbs. 

of force 

here! 

“S” cam 

1 inch in 

length 

100 psi is exerted against a diaphragm 30 sq. ins. in area inside the brake chamber 

pictured here. 30 X 100 = 3000 lbs. of force! Times that by the length of the slack 

adjuster (lever) and the total force is… 18,000 ft-lbs. of force. 

27



• Putting It All Together 

• Review 

28

As the driver applies the brake pedal, the brake linings are forced against the inside 

surface of the brake drum. The spinning wheel produces friction as the linings make 

contact. This friction causes heat which is then dissipated to the atmosphere. The 

amount of heat the drums can absorb, depends on the thickness of the metal. If the 

drums are “turned” or worn too thin, they will be unable to absorb the heat produced 

by braking, and brake fade will occur. 

If one set of 

brakes was 

poorly 

adjusted, the 

rest of the 

brakes would 

have to absorb 

more heat 

energy than 

they were 

initially 

designed for. 

Speed is critical 

when descending a 

steep grade. To 

avoid overtaxing 

the brakes, follow 

this simple rule: 

Never descend a 

hill at a speed 

greater than the 

vehicle is capable 

of climbing the hill. 

29

Review of Section Two 

In this section we have studied the concepts of: 

•Heat, Energy, Traction and Friction 

•Speed, Weight and Distance 

•How we obtain a Mechanical Advantage to achieve Force 

Multiplication 

•Compressing air, and combining this with Force Multiplication 

•Looked at Stopping Distance and how it can be effected by adverse 

conditions. 

The next section will begin our exploration of the components of the 

air brake system. 

Let’s Review... 

30

Review Questions: Section 1 

1) What is the final factor that will determine if a vehicle will 

move? 

2) What is the final factor that will determine if a vehicle will stop? 

3) How is the heat dissipated that is generated by the brakes? 

4) If one set of brakes were poorly adjusted, what effect would this 

have on the remaining sets of brakes? 

5) What is meant by the term friction? 

6) What are the four components of stopping distance? 

7) If the weight of your vehicle were doubled, how many times 

must the braking power be increased in order to stop in the same 

distance? If speed doubles? If both weight and speed were 

doubled? 

8) Brakes stop the vehicle. Is this true or false? 

4)…the 5)…the 8)… False. force other 6)…perception 3)…by Traction resisting brakes the 7)…two determines would brake movement 1) time, Traction 2) drums times; have Traction reaction if to between to four the do the vehicle’s time; more times; atmosphere two brake than eight surfaces brakes their lag; times share will in contact be of effective. braking 

braking distance 

Left click for answers 

31


3. Five Main Components: 

a) Compressor & Governor 

b) Air Lines 

c) Reservoirs 

d) Foot Valve 

e) Foundation Brakes 

32



a) Compressor & Governor 

33

The Compressor & Governor 

The compressor has two adjacent pistons attached to it’s crankshaft which is in 

is in constant drive with the engine. When the compressor is “loaded” or 

pumping, air is forced through the main discharge line to the reservoirs. As 

pressure builds to one of two main standard operating pressures (85 - 105 or 115 

115 - 135) the unloading device in the top of the compressor, holds the inlet 

valves open and air is pumped between the pistons, allowing the compressor to 

compressor to cool. This is the “unloaded” stage. The compressor is controlled 

controlled by the governor which is usually mounted to the side of the 

compressor. A typical “two-flow” compressor is pictured here. 

We need to know... 

• The compressor output can be effected by a dirty air filter, loose belts, or worn rings. 

• It is usually lubricated by the engine lubrication system. (some have their own) 

•During the pre-trip inspection, the compressor is checked visually before engine start-up, 

start-up, for belt condition and tension, mounting security, and evidence of oil leaks. 

•During the pre-trip inspection, the compressor output is checked. It must be able to pump 

pump from 50 to 90 psi. in 3 minutes or less. 

34

The Governor 

The governor may be considered to be the brain of the air brake 

system. It tells the compressor when to load and unload based on 

the amount of air pressure in the reservoirs. Two standard main 

reservoir operating pressures are: 

•85 psi. to 105 psi. and 115 psi to 135 psi. 

•Some systems operate at 85 psi. to 135 psi. 

•The spread between loading and unloading pressures must 

not be less than 20 psi. 

•During the pre-trip inspection, the cut-in (load) and cut-out 

(unload) pressures of the governor are checked. 

…next slide 

35

The Governor 

(continued) 

(1) Air flowing from the compressor moves down the main discharge line to the wet tank. 

(2) From the wet tank, it passes through a one-way check valve to the dry tank 

(3) Reservoir pressure arrives at the governor, “telling” it the pressure in the dry tank 

(4) When sufficient pressure is developed, the governor signals the compressor to unload. 

4 

3 

1 

2 

36

One-way Check Valve 

The one-way check valve is situated between the wet tank and the dry tank. 

This is the first defence against a catastrophic air loss. If the main discharge line 

ruptured or the wet tank were punctured, the one-way check valve will not 

allow the air pressure to flow backwards out of the dry tank. 

When draining the reservoirs, always open the drain valve of the wet tank first. 

If there is still pressure in the dry tank, you know the one-way check valve 

worked. 

The check valve is a common valve in the air brake system and is used at many 

other locations and for different applications. 

37



b) Air Lines 

38

Air Lines 

• Air lines link compressed air from one component to another. 

• Air flowing from the Compressor moves down the Air Line or main 

discharge line to the wet tank. 

• Air lines are made of a durable, tensile material – they can withstand 

pressure of at least 150 PSI. 

• Application air lines (also known as service or control lines) have a 

narrow diameter – Supply air lines (also known as delivery lines) have a 

larger diameter. 

39


3. Basic Components: 

c) Reservoirs (Air Tanks) 

40

Safety Valve 

The Reservoirs 

One-way 

check valve 

Low air warning device 

Wet tank 

Dry Tank 

Drain Valve 

Drain Valve 

The reservoirs, or tanks, are made of steel and serve to contain the compressed air delivered 

by the compressor. Air from the compressor is hot. When it comes into contact with the cold 

steel, condensation occurs. This moisture must not be allowed to accumulate in the tanks. If it 

did, it would reduce the amount of air the tank would be able to hold, and thus, reduce the 

volume of air available for the operation of the vehicles brakes. A safety valve is fitted to allow 

the air to escape if over-pressurisation occurs. (140 to 150 psi.) A one way check valve between 

the reservoirs disallows the back flow of air from the dry tank to the wet tank. A low air 

warning device will warn the operator of low air pressure when the pressure in the reservoirs 

drops to 60 psi. This warning is either audible or visual. 

We need to know 

We need to know…more reservoirs, more volume; they must be drained at least once per day; 

they must be drained completely in order to allow all the moisture and sludge to escape; the 

wet tank should be drained first so the function of the one-way check valve can be tested. 

41

Low Air Warning Device & Safety Valve 

The low air warning device is a simple spring loaded electrical switch that triggers a 

warning in the driver’s compartment. This device must activate before the reservoir air 

pressure drops to 60 psi. The warning can be either audible or visual. This device is 

checked during the pre-trip inspection. Some systems, usually trucking applications, 

use a “wig-wag” This is a metal flag that drops from above the driver’s windshield into 

his/her line of sight. 

The safety valve is fitted to the wet tank. Its purpose is to release excess pressure should 

the governor fail to unload the compressor due to a governor failure or a problem with the 

unloader mechanism in the compressor head. The safety valve should release when 

pressure reaches 140 to 150 psi. 

Low air warn. 

safety valve 

“Wig-Wag” 

42

Air Gauges 

60 

50 

40 

70 

80 

85 

90 

100 

105 

30 

110 

115 

20 

120 

10 135 bye-bye 

Air gauges come in many shapes and sizes, but they 

all have one thing in common. Information. 

A reservoir gauge will advise of the amount of air 

present in the tank(s) at any given time. It is used 

during pre-trip inspections to determine: 

•governor cut-in and cut-out pressures 

•low air warning device function (above 60 psi.) 

•at what pressure the spring brakes apply 

automatically. 

•pressure drop when a full foot valve application 

is made to test for brake adjustment. 

•check for air leaks (maximum 3 psi./min. loss 

single units – buses or tractors, 4 psi./min. 

tractor & trailer, 6 psi./min. tractor & tandem 

trailer) 

•an application gauge will tell you how much air 

is being delivered to the brake chambers. 

43

Air Gauges 

NOTE: Gauges on 

Double Decker and 

Dart read in KPA 

with a scale from 

0 - 11. 

Operating range is 

outside of the red bar 

44



d) Foot Valve 

45

The Foot Valve 

Two typical foot valves are pictured here. The uppermost one is used 

primarily in buses. The lower example is suspended from the firewall and is 

used mostly in trucking applications. They operate in precisely the same 

way, being spring loaded and self-balancing. 

We need to know... 

•The foot valve is considered to be the most important valve in the system, 

because without it, we could not control brake application pressure. 

•It is a self-balancing device, meaning that even if a small leak occurred in the 

system during a brake application, the foot valve would regulate the application 

air such that the brakes would remain applied. 

•The foot valve is spring loaded, so the operator feels only the spring pressure, 

not the pressure of the brake application, as in a hydraulic brake system. 

•Maximum brake application pressure available at any time is only that which 

is present in the system’s reservoirs. Application pressure cannot exceed 

reservoir pressure. 

46

The Foot Valve 

The foot valve controls application pressure. 

•Reservoir air is available at the base of the foot valve. (dark green) 

•Application air, as demanded by the operator, is delivered to the brake 

chambers. (light green and light red) 

•The further down the operator depresses the treadle, the more air 

pressure is delivered to the brake chambers. 

•When the treadle is released, the brake application is released. 

Front 

Application air 

Application air 

Rear 

Reservoir air 

47



e) Foundation Brakes (Brake Chambers, Slack Adjusters, Brake Linings & 

Brake Drums) 

48

The Foundation Brakes: Brake Chamber, Slack Adjusters, 

and Brake Linings /Drums 

A brake chamber is a circular container divided in the middle by 

a flexible diaphragm. Air pressure pushing against the 

diaphragm causes it to move away from the pressure, forcing the 

push rod outward against the slack adjuster. The force exerted 

by this motion depends on air pressure and diaphragm size. 

The Slack Adjuster has two functions in the operation of the 

vehicle's brakes. First, it converts the pushing motion of the push 

rod into a twisting motion at the “S” cam. Second, as the name 

suggests, the slack adjuster provides a means of reducing “free 

play” or slack in the foundation brake linkages. 

49

Two Types of Slack Adjusters 

There are two types of slack adjusters currently 

in use with drum type brakes: 

manual adjustment 

automatic slack adjustment 

Manual adjustment must be 

made every day at the start 

of the shift. If conditions 

warrant, adjustment may be 

required more often. 

Pre-Trip Inspection: 

•Set up brakes before the start of each day’s 

shift. 

•Maximum push rod travel under a full foot 

valve application must not exceed 1 3/4 

inches. 

•Maximum push rod travel under pry bar 

pull must not exceed 3/4 of an inch. 

•When making a full foot valve application 

with engine off and park brake released, look 

for any large drops in air pressure as 

indicated on the pressure gauge. 

Automatic slack adjusters can 

be adjusted by simply making a 

full brake application with the 

foot valve. Upon release of the 

foot valve, the slack adjuster 

will automatically adjust your 

brakes to optimum push rod 

travel. 

50

Automatic Slack Adjusters 

Pre-Trip Inspection: 

•Set up brakes before the start of each day’s shift. 

•Maximum push rod travel with a type 30 brake chamber under a full foot 

valve application must not exceed 2 inches - using the pry bar method, there 

should be no more than ¾of an inch of push rod travel. 

•When making a full foot valve application with engine off and park brake 

released, look for any large drops in air pressure as indicated on the 

pressure gauge. 

51

Automatic Slack Adjusters 

Automatic Slack Adjusters With Hexagonal Adjusting Bolts: 

•If the slack adjuster has a hexagonal adjusting bolt, the 

brakes are adjusted by turning the adjusting bolt in a 

clockwise direction until the lining contacts the drum. 

•Baking off the adjusting bolt by 1/2 turn should restore 

running clearance (backing off may take considerable force 

and a ratcheting sound and feel will occur – this is normal). 

Automatic Slack Adjusters With Square Adjusting Bolts: 

•If the slack adjuster has a square adjusting bolt located at the bottom end of the 

body, do not attempt adjusting until a spring-loaded pawl that meshes with 

internal teeth is disengaged. 

•If the slack adjuster has a square adjusting bolt, the brakes are adjusted by 

turning the adjusting bolt in a counter-clockwise direction until the lining 

contacts the drum. 

•Baking off the adjusting bolt by 1/2 turn should restore running clearance. 

•Release the button or re-install the spring and pawl if they were removed. 

52

Brake Chambers and Slack Adjusters 

As the linings wear, the distance 

between the linings and the drums 

begins to enlarge. 

If this condition were to deteriorate, 

braking could be lost completely! 

Loss of brake adjustment is the 

leading cause of brake failure in the 

transportation industry! 

53

Brake Linings and Drums (or Rotors) 

The assembly pictured to the left, is the 

foundation brake. It is composed of the brake 

drum, linings, shoes, “S” cam, slack adjuster, 

push rod, and brake chamber. 

•The drum dissipates the heat generated by the 

brakes. 

•The linings are made of composite material and 

wear according to use. (load, speed, frequency of 

braking) 

•The distance between the lining and the drum 

when the brake is released will become larger as 

the linings wear. 

•The slack adjuster provides a means of 

reducing this distance. 

Remember… 

The most common cause of brake failure in the commercial transportation rtation industry, is 

lack of proper adjustment! Always check brake adjustment, or set up your brakes, 

during your initial pre-trip inspection. 

54



• Review 

55

Review of Section Three 

In this section we learned… 

The 5 main components of the air brake system… 

•Compressor to pump air, with a governor to control the compressor 

•Air Lines to allow the pressurized air to flow between the brake system 

components 

•Reservoirs to store the compressed air 

•Foot Valve (usually called a brake pedal ) to apply the brakes by directing 

compressed air from the reservoir to the brakes 

•Foundation brakes, including brake chambers, slack adjusters, brake linings 

and drums or rotors, to transfer the force generated by the compressed air through 

mechanical linkage to apply the brakes 

We know the purpose of each of these components within the air brake system, and we 

achieved a basic understanding of how each one works. 

In the next section we will begin to put these components together to build a basic 

system. 

Let’s review... 

56

Review Questions: Section Three 

1) What are the five main components of the air brake system? 

2) How is a plugged air filter likely to effect the compressor? 

3) What causes moisture to form in the air brake system? 

4) When is the compressor able to accomplish most of its cooling? 

5) How often must reservoirs be drained? 

6) What is the maximum air pressure available for a brake application at any given time? 

7) Is it necessary to allow all the air to escape from the reservoirs? Why? 

8) What would result if the brake drums were allowed to wear thin? 

9) What are two functions of a slack adjuster? 

10) How does the amount of slack effect the operation of the brakes? 

11) What is the maximum allowable push rod travel under 

a full brake application? With a pry bar? 

12) What is the most common cause of loss of effective braking on air brake equipped 

vehicles? 

13) What causes brake fade at high brake temperatures? 

13) 

7)Yes. Otherwise the sludge 1) 2) restrict compressor/governor; will 8) they not 6) 11) air run would that 10) flow one 4) out which increased during and overheat because reducing three is the present air brake the resulting quarter unloaded lines; compressor air lag under reservoirs; the inches; time; stage 

loss pressure output possible three of foot braking quarters will valve; complete push foundation of it an loss to inch the sides brakes 

9) 5) 3) 

drums of effective braking 

convert daily condensation 

worn 

or a more 

thin 

pushing often 

or turned 12) brakes out of adjustment 

motion if conditions 

to far 

to a twisting warrant motion; allow a means of adjusting the brakes 

Left click for answers 

57


4. Basic Air Brake System: 

a) Review How 5 Main Components Work Together 

b) Quick Release Valve Function 

c) Relay Valve Function 

d) Park Brake System 

Please note: the piping diagrams that follow, do not accurately represent the actual 

valves and components in an air brake system. Our purpose is to provide you with 

a basic, theoretical understanding of air brake systems sufficient for you to obtain 

an air brake endorsement on your license, and perform a pre-trip inspection so as 

to diagnose problems in the system before going on the road. 

58



a) Review How 5 Main Components Work Together 

59

No. 1 

COMPRESSOR: pumps air; 50 to 90 psi. in less than 3 minutes; lubricated by engine oil; 

belt driven (some have their own lubrication and drive system); has an air filter, that if 

plugged, would reduce efficiency. 

Left click for answer 

60

No. 2 

GOVERNOR: tells compressor when to load and unload (85- 

105 or 115 to 135 psi.) 


61

No. 3 

FOOT VALVE: most important valve in the system; spring loaded 

and self balancing; has a different feel from hydraulic brakes 


62

No. 4 

AIR LINES: link air from one component to another. 

RESERVOIRS: contain compressed air; must be drained daily and completely; more 

than one reservoir to provide a greater volume of air. 


63

No. 10 

BRAKE DRUM: absorbs and dissipates heat generated by braking. 


64

No. 11 

BRAKE SHOES AND LININGS: shoes contain linings; linings make contact with inside 

surface of brake drum, creating friction required to provide braking. 


65

No. 12 

“S” CAM: forces shoes and linings against drum 


66

No. 13 

SLACK ADJUSTER: provides a means of reducing the distance between 

linings and drums (slack); converts pushing motion into twisting motion 


67

No. 14 

PUSH ROD: transmits force from brake chamber; must not move more than 1 

3/4 inches under full application test: 3/4 inch under pry bar pull 


68

No. 15 

BRAKE CHAMBER: part of force multiplication process; drives push rod 


69



b) Quick Release Valve Function 

70

Quick Release Valve and Stop Light Switch 

The Quick Release Valve, situated on the front axle, provides 

a quicker release of the front brakes by allowing the air to 

exhaust at the centre point between the brake chambers. The 

Stop Light Switch can be activated with as little as 4 psi 

brake application pressure. 

Quick release valve 

Stop Light Switch 

71



c) Relay Valve Function 

72

The Relay Valve 

The relay valve provides quicker application and release of the 

rear brakes. 

The relay valve function is used in other applications within the 

air brake system, which will be explored later. 

In order to understand the need for this valve, let’s look again at 

brake lag time… next slide 

73

Brake Lag Time 

...is the time it takes for air to travel through a properly maintained system. 

high volume - slow moving 

low volume - fast moving 

•Lag time is dependent, however, on the inside diameter of the brake line through 

which the air is travelling. 

• In the two examples pictured here, the air will travel faster through the smaller 

diameter line and slower through the larger diameter line. 

• However, the larger diameter line is capable of delivering a much greater volume of 

air, as opposed to the smaller line which delivers a much smaller volume of air. 

Click for animation (once for each pipe) 

74

In this diagram, the dark green line is a LARGER diameter than the bright 

green line. The dark green line delivers reservoir air to the foot valve AND to 

the relay valve at the rear axle. When a brake application is made, the foot 

valve delivers the requested amount of air to the relay valve, through the 

bright green line, known as a “control line.” 

The air delivered by the foot valve “dead-ends” at the relay valve. This causes 

the relay valve to “open” and it delivers the requested amount of application 

air from its own supply of reservoir air. 

...next slide 

Control line 

Reservoir air 

75

Relay Valve Operation 

Control line from foot valve 

Air dead ends here! 

To rear brake chambers 

Application air from the 

foot valve arrives at the top 

of the relay valve, via the 

control line (bright green.) 

It contacts a diaphragm 

inside the relay valve, and 

dead ends there. 

This opens the relay valve 

internally, and air from the 

reservoir is metered to the 

brake chambers. 

From Reservoir 

...next slide 

76

• Because the relay valve accesses the 

reservoir air directly, and the application air 

acts only as a control device, the brake lag 

that would normally occur between the foot 

valve and the rear brakes, is greatly reduced. 

• This concept of “relaying” a brake 

application is used in other valves in the 

system, so it’s important to confirm your 

understanding at this point. 

77

• If the relay valve were not installed in the 

system, the front brakes would be applying 

before the rear brakes. (The foot valve is closer 

to the front brakes, than the rear brakes.) 

• Since most braking is accomplished by the 

rear brakes of a long wheel base vehicle, this 

would not be a desirable situation. 

78



d) Park Brake System 

79

Section - Parking Brakes 

By the end of this section the student will understand: 

• How a park brake works. 

• Why a spring brake is a reliable type of park brake. 

• How to release different types of park brakes when 

the air system cannot supply sufficient pressure for 

release. 

80

A spring type park brake serves three important functions: 

•Its primary function is to provide a reliable means of 

securing an air brake equipped vehicle. (parking brake) 

•It can serve as an emergency brake if an air loss 

occurred during vehicle operation. 

•It can be used in place of the air operated primary 

service brakes in a dual air equipped vehicle. (We will 

study dual air systems later in the air brake course.) 

81

The park brake control 

valve, located in the 

driver’s compartment, is 

standardised throughout 

the commercial transport 

industry. 

The valve knob will 

always be diamond 

shaped and yellow in 

colour. 

82

However, some valves 

may be designed to be 

pushed to apply the 

brake, while others may 

be designed to be pulled 

out to apply the brake. 

Buses usually incorporate 

the first type where the 

valve is pushed to apply 

the park brake. 

83

The Parts of the Spring Type Parking Brake 

Push Rod 

Service brake 

chamber 

Service brake 

diaphragm 

Spring brake 

diaphragm 

Spring brake 

chamber 

Park brake 

actuating spring 

Access for 

caging bolt 

Slack Adjuster 

Note: on “Maxibrake” 

type park 

brakes there is no 

access for caging. 

An emergency 

release reservoir 

is provided 

instead. 

Return springs 

84

Park Brake and Service Brake Released 

Service brake is released (no air 

present in service brake chamber) 

System air pressure is present 

in spring brake chamber. 

The park brake spring is 

held compressed by the 

system air pressure. The 

park brake is released, 

but ready for emergency 

braking if required. 

85

Park Brake Released - Service Brake Application 

System air is still 

present in the park 

brake chamber. The 

park brake spring is 

compressed so the park 

brake is released. 

Application air, as delivered by the foot valve, is present in 

the service brake chamber. The service brakes are applied. 

86

Park Brake Applied - Service Brake Released (no compounding) 

No air in service 

brake chamber 

No system air pressure in 

park brake chamber 

The park brake spring 

has expanded, pushing 

against the push rod, 

which has moved the 

slack adjuster away from 

the brake chamber, and 

the brakes are applied. 

87

Mechanical Release of the Park Brake (Caging the Spring) 

On “piggy-back” park brake systems such as the ones we’ve 

looked at, it may be necessary to release the park brake after an 

emergency application due to some sort of air system failure. Here, 

the caging bolt has been inserted into the park brake chamber 

through the access hole previously described. The operator would 

simply turn the bolt until the park brake spring . is compressed and 

the brakes are released. 

Caging Bolt inserted into 

spring brake chamber 

88

Mechanical Release of the Park Brake (Caging the Spring) 

The vehicle must be blocked before beginning the caging 

operation. The service brake and the park brake will not 

function because the caging bolt renders the push rod 

immovable. 

Note: “Maxi-Brake” 

. 

park brakes used on 

buses (vehicles with air 

suspensions) utilise an 

“Emergency Release 

reservoir” to release the 

park brake, and cannot 

be caged in this manner. 

Caging Bolt inserted into spring brake chamber 

89

“Maxi-Brake” type Park Brake 

The “Maxi-Brake” parking brake system is 

somewhat different in design. A typical 

maxi-brake chamber is pictured here. 

Note there is no access hole in the end of the 

chamber to allow caging of the brake. 

Rather, the maxi-brake uses a either a 

single control valve that would not permit 

an emergency release of the park brake, or 

a dual control valve that allows access to an 

“Emergency Release Reservoir.” 

90

“Maxi-Brake” type Park Brake 

The emergency release reservoir serves 

the same purpose as caging. 

It will allow the operator to move the 

vehicle a short distance (usually to the 

side of the road) in the event a failure of 

the main system caused an emergency 

application of the park brake. 

Apart from these differences, the maxibrake 

works in exactly the same way as 

the “piggy-back” brakes seen earlier. 

91

“Maxi-Brake” Operation (single control valve.) 

When the park brake control valve is opened, the system air (green) 

flows through to the park brake chambers compressing the springs. The 

park brake is released. 

Park 

brake 

chambers 

System air 

Left click to begin animation 

92

“Maxi-Brake” Operation (single control valve) 

When the park brake control valve is closed, this exhausts the air from 

the spring brake chambers, and the park brake is applied. 

Park brake 

chambers 


93

Park brake systems with double control valves... 

Some systems utilize an extra reservoir that allows the 

release of the park brake even if the main reservoir were 

completely emptied due to failure of the system. 

Some systems, usually on trucking applications, would 

require the operator to physically crawl under the unit 

and “cage” the spring brakes if the vehicle needed to be 

moved to a safer location. (This procedure will be 

demonstrated by your instructor during this course.) 

94

Park brake systems with double control valves... 

Buses use an air suspension system, so crawling under 

the vehicle would not be safe due to the possible sudden 

loss of air that could occur. 

The next slide depicts a system into which an isolated 

reservoir has been piped. 

The purpose of this “Emergency Release Reservoir” is 

to allow the operator to move the vehicle to a safer 

location until repairs could be affected. 

95

“Maxi-Brake” Operation (Emergency Release Reservoir) 

If a loss of system air occurs, and the operator needs to move the vehicle a short 

distance to safety, opening the park brake control valve and the emergency release 

reservoir valve at the same time, will allow the air in the emergency release 

reservoir to flow through to the spring brakes, holding them in release. 

Foot valve / service brakes 

NOT operational. Maxi must 

be re-applied to stop the 

vehicle 

Emergency 

release 

reservoir 

Air loss from main system 

Double control 

valves 


96

Park Brake Emergency Functions... 

1. The park brake control valve will close automatically 

whenever system air pressure drops to between 45 and 20 

PSI. (At BC Transit, most apply around 50-60 p.s.i.) 

2. Some systems use a piping arrangement that requires the 

operator to close the valve manually if system air were lost. 

It will not close and apply the spring brakes automatically. 

3. When the spring brakes apply automatically while the 

vehicle is in motion, the operator can expect the rear wheels 

of the vehicle to virtually “lock-up.” If road conditions were 

less than ideal (frost, snow, ice etc.) a skid to loss of control 

can be anticipated. 

97

Park Brake Emergency Functions... 

4. When operating a vehicle where there is no emergency release 

reservoir (most tractor-trailers and trucks) never go under the 

vehicle to wind off (cage) the spring brakes without first 

verifying that the vehicle cannot roll away when the brakes 

release. Block the vehicle! 

5. The effectiveness of the spring brakes is entirely dependent upon 

brake adjustment. If your brakes are not properly adjusted, your 

park brake may be unreliable or even useless. Remember to 

check for slack brakes and set them up... OFTEN!!! 

6. Applying the service brake when the park brake is applied can 

damage the brake components. (push rods, slack adjusters.) 

Therefore, never make full service applications when the park 

brake is applied. This is known as compounding the brakes. 

98



• Review 

99

No. 5 

ONE - WAY CHECK VALVE: allows 

flow of air in one direction only 


100

No. 6 

LOW AIR WARNING DEVICE: activates by 60 

psi.; needs to be audible or visual 


101

No. 7 

SAFETY VALVE: releases excess pressure in the reservoir due to governor 

failure or compressor unloader mechanism failure. Releases at 140 to 150 psi. 


102

No. 8 

RELAY VALVE: provides quicker 

application and release of the rear brakes. 


103

Review of Section Four 

1) Why are springs brakes a reliable type of parking 

brake? 

2)What is meant by “compounding the brakes?” 

3) How are spring brakes held in the released position? 

4) What are the functions of the cab- mounted park brake 

control valve? 

5) Will park brakes apply automatically in ALL braking 

systems? 

2) 1) Application They are applied of service by spring brakes pressure, and park not 

4) brake by air at pressure. the same time 

3) apply By system and 5) release No. Some air pressure 

the types park brake require the operator to apply the 

park brake. 

Left click for answers. 

104

Review of Section Four 

6) Why is it important to release the park brake before 

making a full brake application test? 

7) What is the purpose of an “emergency release reservoir” 

in a parking brake system? 

8) How can some types of spring brakes be released 

without the use of air pressure? 

9) Why should a spring brake be disassembled by qualified 

personnel only? 

6) So as to not compound the brakes 

9) The 7)to allow the operator to release the park brake when 

8) by spring caging insufficient is under or winding extreme air is them available pressure off by for and hand the could task. 

fly out of 

the chamber causing severe injury or even death. 


105


5. Other Air Brake System Components: 

a) Air Dryers, Alcohol Evaporators & Injectors 

b) Automatic Drain Valve (Spitter Valve) 

c) Front Wheel Limiting Systems 

d) ABS (Anti Lock Braking) & ATC (Automatic Traction Control) 

e) Long Stroke Pushrods 

f) Disc Brakes 

g) Wedge Brakes 

106



a) Air Dryers, Alcohol Evaporators & Injectors

Air Dryers, Alcohol Evaporators & Alcohol Injectors 

Air dryers are optional devices that are installed in the compressor discharge line between the compressor and the first 

reservoir. They are designed to remove any water vapour, oil mist and carbon particles from the air before it is 

delivered to the first reservoir. 

Alcohol evaporators and alcohol injectors are optional devices that introduce a small amount of alcohol vapour into 

the air system – in effect, they lower the freezing point of any moisture that has collected in the air system. 

Front 

Rear 

Reservoir air 

Air Dryer 

108



b) Automatic Drain Valve (Spitter Valve)

Automatic Drain Valve (“Spitter( 

Spitter” Valve) 

Safety Valve 

Low air warning device 

Wet tank 

Dry Tank 

Automatic 

Drain Valve 

Drain Valve 

Drain Valve 

The Automatic Drain Valves (“Spitter” Valve) are optional devices which aid in reducing the 

amount of moisture and contaminants that collect in the air brake system. These valves are 

installed on some or all of the reservoirs, or connected to the compressor/governor, or the foot 

valve on some air brake systems. 

Most are self-contained and momentarily open each time reservoir pressure falls (typically 

following a brake application) or each time the compressor cycles. 

Some Automatic Drain Valves are equipped with an electric heating element to prevent 

freezing in cold weather. 

110



c) Front Wheel Limiting Systems

Front Wheel Limiting Valve 

The Front Wheel Limiting Valve allows the operator to reduce brake 

pressure to the front brake chambers by 50% (some automatic types use a 

graduated reduction that provides no limiting after 60 psi. of application air 

is reached). 

Front wheel 

limiting valve 

Cab switch 

112



d) ABS (Anti Lock Braking) & ATC (Automatic Traction Control)

ABS (Anti Lock Air Brake Systems) 

• ABS are typically made up of 3 main sections: 

speed sensing, decision-making, and brake releasing or modulation. 

• As the wheels rotate, vehicle speed is detected by the sensors which 

delivers a pulsating current monitored by an ECU (Electronic Control 

Unit). 

• With normal braking, the sensors will detect a graduated decrease in 

pulsating current, with no change to braking. 

• If the ECU detects a sudden change (wheel lockup) in the pulsating 

current, the ABS system will activate, thereby signaling a release of 

air from the brakes (modulation). 

• As the brakes begin to release, the wheels will regain traction, the 

pulsating current will be restored, and the ECU will allow the brakes to 

re-apply. 

• If the lockup re-occurs the apply-and-release cycle will repeat as 

often as necessary – most systems are capable of cycling the brakes 

up to five times per second. 

114

ATC (Automatic Traction Control) 

• Traction control systems are designed to prevent wheel spin 

in the power mode. 

•Traction control electronics are integrated into the ABS ECU 

(Electronic Control Unit). 

•Traction control attempts to regain traction by braking the 

spinning wheels, and sometimes throttling back engine 

power. 

•Unlike an ABS, traction control can automatically apply the 

brakes – the driver does not need to depress the brake pedal 

for traction control to engage. 

• As wheel speed balance is regained, traction is stabilized, 

preventing spin or jackknifing. 

•Traction control is especially valuable when a light drive 

wheel load might allow the wheels to spin under power, or 

when a tractor is pulling multiple trailers. 

115



e) Long Stroke Pushrods

Long Stroke Push Rods 

Many new air brake systems are equipped with long stroke 

brake chambers. As the name implies, a long stroke 

chamber design has a longer pushrod stroke than the pushrod 

of a standard brake chamber. 

The key advantage of a longer stroke is that it keeps 

brakes in adjustment longer (it does not create more 

braking force). Note: long stroke push rods should not 

be used with regular brake chambers – this could cause 

poor brake balance and timing. 

Long stroke brake chambers can usually be identified by 

square-shaped inlet ports or a name tag on a clamp bolt. 

117



f) Disc Brakes

Disc Brakes 

Callipers 

Rotor 

…operate differently from drum brakes. 

Rather than using shoes and linings that 

move outward toward the inner drum 

surface, disc brakes use a calliper or “C” 

clamp principal. 

In the representation pictured here, the 

callipers are at the top of the rotor. When 

the operator applies the brake, a power 

screw device forces the callipers together, 

sandwiching the rotor between them. The 

rotor is connected to the wheel and tires, 

and so braking occurs. 

The adjustment of this type of brake is 

quite different from that used for drum 

brakes. Check manufacturer’s 

instructions if adjusting disc brakes. 

119



g) Wedge Brakes

Wedge Brakes 

This type of brake uses one or two small air chambers with 

wedge-shaped pushrods. Once quite common on drive and 

trailer axles, wedge brakes are now usually found only on 

steering axles. 

When the brakes are applied, air pressure in the brake 

chamber pushes the wedge part of the pushrod between two 

rollers, forcing the brake linings out to contact the brake 

drum. 

Unlike conventional s-cam braking systems, drivers cannot 

easily check the wedge brake adjustment. While most 

wedge brakes have internal automatic adjusters, if the wedge 

brakes need to be checked for adjustment, or repairs are 

needed, only a qualified mechanic should carry this out. 

121


6. Single Circuit Tractor Trailer Systems:

Section Six 

Single Circuit Tractor Trailer Systems 

By the end of this section the student will be able to understand: 

•How the tractor and trailer brakes connect and work together 

•The operation of the various valves and systems 

•The operation of the safety devices incorporated into the system 

•Emergency functions of the system 

•How to diagnose some faults associated with the connection and 

functioning of the trailer brakes 

Note: Single circuit systems only are covered here. Dual air systems will 

be dealt with later in the course. As such, the trailers depicted in this 

section are NOT equipped with spring brakes. 

123

A typical tractor-trailer trailer unit connected and ready for work. 

The unit is called a semi-trailer because the 

towing vehicle, or tractor, actually carries part 

of the load. The trailer has no front axle and 

would fall on it’s nose if it disconnected from 

the tractor. 

The tractor pictured here is a tandem axle. (It 

has two sets of driving axles.) Some have only 

one and are called “single axle tractors.” 

124

Parts of the Trailer (van style) 

1) Wheel blocks: trailers not equipped with spring brakes, must be blocked 

whenever they are to be left alone for more than 15 minutes. After that time, it can 

be assumed all air has bled away, and no brakes are working. 

2) Crank: allows the trailer to be raised or lowered to meet the height 

requirements of the towing tractor. It has two speeds or “gears.” Push or pull the 

handle to change gears. 

3) “King Pin”: a steel pin that fits into the “fifth wheel” of the tractor. It connects 

the two units and the trailer swivels around it. 

4) The air line 

connectors and cable to 

carry power to the lights 

and turn signals. 

5) Landing gear: 

supports the 

trailer when 

disconnected. 

4 

3 

2 

5 

1 

125

The Parts of the Tractor 

(1) The frame rails or chassis 

(2) the tandem drive axles, 

(3) the “Fifth Wheel” which bears the load of the trailer 

(4) the air lines and power cable 

(5) mud flaps (must always be attached to the tractor frame when “bobtailing”- running 

without a trailer connected to the tractor) 

4 

3 

1 

2 

5 

126

The “Fifth Wheel” 

The fifth wheel is located on 

the centre of the frame rails 

directly behind the cab of the 

tractor. 

The release handle is used to 

open the locking jaws prior to 

connecting to or disconnecting 

from the trailer. The coupler 

arm helps the king pin of the 

trailer to guide itself into the 

locking jaws. 

The pivot point allows the fifth 

wheel to move up and down 

with the trailer as the coupled 

unit moves over uneven 

roadways and hills. 

127

Preparing to Connect Tractor and Trailer (Step 1) 

Positioning the tractor correctly is important in order to align the king pin of the trailer 

with the jaws of the fifth wheel. Back straight up to the trailer, not at an angle. Bring the 

tractor to the point where the fifth wheel just makes contact with the apron of the trailer. 

Then stop, set the park brake and step out to make a visual inspection. 

Fifth wheel 

Trailer apron 

128

Hooking Up (Step 2) 

A visual inspection by the operator before backing under the the trailer will 

confirm... 

1) The trailer height matches that of the fifth wheel 

2) The fifth wheel release handle is “open”(jaws are open to accept the king pin.) 

3) The trailer is blocked and secure. 

Once these steps have 

been verified, then it’s 

time to connect and 

charge the trailer air 

system. DO NOT 

attempt to back under 

the trailer until the air 

connections have been 

made. Even when 

blocked, the jolt may be 

enough to move the 

trailer over the wheel 

blocks and set it rolling. 

129

Connecting the Trailer Air Lines and Electrical Cable 

(Step 3) 

“Glad Hand” couplers are used to connect the air lines of the tractor and the trailer. 

•They are identical in design with no left or right. (they may be coloured red and green) 

•They are firmly attached to the front of the trailer and are at the ends of the air lines leading 

away from the rear of the tractor behind the cab. 

•A rubber seal provides an air tight fit (it should be checked for wear) 

•A screen inside the opening prevents grit from entering the system. (check it, too.) 

•To join them together, hold them face to face at a 90 degree angle to each other. Snap down 

and turn them until they lock together. 

•When not in use (bobtailing) they should be coupled to the “dead end” couplers on the back 

of the tractor so they remain clean. 

•Remember to plug in the power cable for the lights and signals on the trailer. 

130

Charging the Trailer Brake Reservoir (Step 4) 

TRAILER 

SUPPLY 

SYSTEM 

PARK 

The Trailer Supply Valve mounted on the dash board of the tractor next to 

the park brake control valve, allows the operator to “charge” the trailer 

reservoir. By pulling out or pushing in this valve, depending upon the 

design, the system air pressure in the tractor will flow through to the 

trailer thus supplying the trailer brake system with air pressure equal to 

the tractor’s. 

131

Completing the Coupling Procedure (Step 5) 

The air lines have been connected. (red: supply/delivery; green: service/control) 

The blue line is the electrical connection supplying power for the trailer lights and 

turn signals. 

The operator has opened the trailer supply valve and the trailer reservoir has been 

filled with system air. 

The operator now closes the trailer supply valve which “dynamites” (full on) the 

trailer brakes. 

The operator may now select a reverse gear and slowly back under the trailer until 

the king pin locks securely into the 

jaws of the fifth wheel. 

With the landing gear of the trailer 

still supporting the front of the 

trailer and the trailer brakes still 

dynamited, a tug test is performed to 

make sure the jaws of the fifth wheel 

locked around the king pin of the 

trailer. 

The unit is now ready for it’s pretrip 

inspection. 

132

The Semi-Trailer Ready for its Pre-trip Inspection 

The publication “Driving Commercial Vehicles - A Guide For Professional Drivers”Chapter 10 

Page 227, details the pre-trip inspection required for a semi-trailer. Some key points: 

•Ensure all lights and signals are working. 

•Visually inspect fifth wheel for security. 

•Raise landing gear before moving the unit. 

•Check trailer brakes are working correctly by applying hand valve while rolling slowly. 

•Perform the three tests required to be sure trailer brakes “dynamite” when required. 

133

Bobtail Proportioning Systems 

Because a bobtail tractor has very little weight over the rear drive axles, it is very easy to lock 

up the rear brakes, even with a light brake application. To help prevent this unwanted lockup, 

and to increase control, some tractors are fitted with a bobtail proportioning system: 

•The system consists of two valves – one controls the steering axle brakes and the other 

the drive axle brakes. 

•When the tractor has a trailer attached, the tractor brakes work normally. 

•When bobtailing, the braking pressure to the drive brakes is reduced by as much as 

75%, preventing the drive axle brakes from locking. 

•At the same time, the steering axle brakes receive full application pressure. 

•A tractor fitted with a bobtail proportioning system will stop in a shorter distance and 

control will be increased, especially on wet or slippery road conditions. 

134

Tractor-Trailer Trailer Air Brake Systems 

The following slides will walk you through the air system, valve by valve 

and line by line. Please remember, these diagrams are not truly 

representative of the actual air piping arrangements. They are, however, 

taken from the “British Columbia Air Brake Manual” so that they will, 

perhaps, look somewhat familiar. So, let’s get started! 

135

Pre-1975 Tractor Trailer Air System (no front brakes) 

This diagram depicts a single circuit air brake system in a typical tractor. 

There are no front brakes pictured since they were not required prior to 

1975. Since the Canadian Motor Vehicle Safety Standard (dual air) was 

introduced, all vehicles built since 1975 must be equipped with front 

brakes. 

136

Introduction to Valves and Components 

The main components of the air system have been addressed earlier in the 

course. We will begin by describing the function of the valves and 

components, some of which have already been mentioned during the “trailer 

connection procedures.” 

137

The Trailer Supply Valve 

The Trailer Supply Valve is a red, octagonal shaped knob located on the dash board of the 

tractor. When opened (pushed or pulled depending on the design) allows tractor system air to 

flow through to the trailer reservoir. In this diagram, if the trailer supply were opened, the air 

would be free to flow out to the atmosphere, as no trailer is currently connected. Once the air 

bled down to between 45 psi and 20 psi., the valve would close automatically. 

TRAILER 

SUPPLY 

Some older trailer supply valves do not close 

automatically. In an emergency, the operator 

must close this type of valve manually. 

Left click to demo air loss 

138

The Hand Valve 

The Hand Valve is mounted on the steering column on the right, the hand valve allows 

independent application of the trailer brakes. Should the trailer begin to slip to the right 

or left as the unit moves along a roadway, the operator can apply greater braking effort to 

the trailer wheels which helps to keep the unit straight. 

Hand valves are spring loaded and will return to the fully 

released position on their own. NOT to be used for parking! 

139

The Two-Way Check Valve 

The two-way check valve will deliver application air either from the hand valve or 

the foot valve to the trailer brakes. Which source delivered, depends on which source 

has the higher pressure. If 20 psi were delivered from the hand valve, but only 10 psi 

were delivered from the foot valve, the 20 psi from the hand valve would ultimately 

be delivered to the trailer brakes. However, if 20 psi were sent from the foot valve, 

and only 10 psi from the hand valve, the tractor and trailer would receive the same 20 

psi brake application. 

20 psi. 

10 psi. 

Left click for animation 

140

The Tractor Protection Valve 

The Tractor Protection Valve is mounted on the rear of the tractor. This valve protects the 

tractor from air loss when the tractor is travelling without a trailer. Any foot or hand valve 

application made by the operator “dead ends”at the top of the valve and thereby cannot 

escape to the atmosphere. It also functions as a relay valve when a trailer is connected and a 

service brake application to the trailer is made. To remember its function, simply recall “the 

tractor protection valve, protects tractor air.” 

141

Foot Valve Application (no trailer connected) 

When a foot valve application is made with no trailer connected, the tractor 

brakes respond normally. Because the Tractor Protection Valve is a type of 

relay valve, this application air from the foot valve, will be “dead-ended” at 

the top of the valve. Therefore, the application air cannot escape from the 

tractor. “Tractor Protection Valve”- “Protects Tractor Air.” 

Left click to start animation 

142

Accidental Hand Valve Application (no trailer connected) 

In this case, the operator accidentally applied the hand valve when no trailer was 

connected. The application air travelled to the 2-way check valve where it was directed to 

the tractor protection valve. (no foot valve application) The air dead ended at the tractor 

protection valve, and no air loss occurred. 


143

Adding the Trailer System 

Here, the trailer has been connected to the tractor, however, the trailer brakes 

are not charged. (no air in the trailer reservoir.) 

We will continue to walk through the system describing the valves and lines as 

we go. 

Later, we will put the system into operation and examine the functions of the 

valves and lines we learned about. 

144

The Trailer Supply /Delivery Line 

The Supply Line connects the tractor protection valve to the 

emergency relay valve near the trailer reservoir. Under normal 

operation the supply line holds whatever air pressure may be 

present in the system at any given time. (reservoir air) 

Supply/Delivery Line 

145

The Trailer Service /Control Line 

The Service Line connects the tractor protection valve with the 

emergency relay valve in much the same way as the supply line. 

However, the service line carries only application air as delivered 

by the hand or the foot valve. If no brake application is being made, 

there is no air the service line. The service line is a type of control 

line. 

Service/Control Line 

146

The Emergency Relay Valve 

The Emergency Relay Valve is located near the trailer reservoir. It is the last valve the 

reservoir air flows through prior to entering the trailer reservoir. Therefore, this valve is 

considered to be the valve that supplies air to the trailer reservoir. The emergency relay 

valve has three important functions: 

1) dynamites the trailer brakes if the trailer breaks away from the tractor or whenever the 

supply line ruptures or loses its air. 

2) relay valve function (quicker application and release of trailer brakes) 

3) one-way check valve prevents loss of air from trailer reservoir if supply line ruptures. 

147

Charging the Trailer Reservoir 

When trailer supply valve is opened, reservoir air from the tractor is 

allowed to flow through a line into the tractor protection valve. The air 

flows freely through the tractor protection valve, along the supply line, 

through the one-way check valve in the emergency relay valve and into the 

trailer reservoir. At this point the trailer brakes are released, and the unit 

is ready to go. 


148

Foot Valve Application (trailer connected) 

Application of the foot valve will apply all the brakes on the tractor and the trailer to the 

same extent. (provided the brakes are properly adjusted.) As before, application air from 

the foot valve (control line air) travels to the two-way check valve. Because no hand valve 

application is being made, the air continues to the top of the tractor protection valve. (deadending 

there.) Air from the supply line is then “relayed” by the tractor protection valve, 

down the service line, dead-ending at the emergency relay valve. Again, the application air is 

relayed. Air is taken from the trailer reservoir and applied to the trailer brakes in 

accordance with the amount requested by the foot valve. 


149

Hand Valve Application (trailer connected) 

This time, the operator has elected to apply the hand valve only. (independent 

application of the trailer brakes.) As before, the application air dead-ends at 

the tractor protection valve. The tractor protection valve relays the request by 

sending the requested air pressure from the supply line, down the service line. 

Again, the air “dead-ends” at the emergency relay valve, which relays the 

required pressure from the trailer reservoir to the trailer brake chambers. 


150

Emergency Functions 

The following slides will walk you through the functions of the tractortrailer 

braking system when things go wrong. For example: 

•The trailer could break away from the tractor if, for example, the king 

pin or fifth wheel failed. 

•A sudden air loss due to a rupture of the supply line will cause the 

trailer to “dynamite.” 

•A rupture to the service line would render the trailer brakes 

inoperative. 

•The supply and service lines could be inadvertently connected 

incorrectly (crossed) 

•Some of these scenarios could be corrected by the operator (glad 

hands simply came apart). Others, would require the services of a 

mechanic. 

151

Trailer Break-Away 

In this case the trailer has completely parted company from the tractor: 

1) The Emergency Relay Valve will “sense” the loss of air in the supply line due to the trailer’s 

separation from the tractor. It will immediately “dynamite” the trailer brakes, meaning that all 

air in the trailer reservoir will be dumped onto the trailer brakes. The one-way check valve 

prevents any loss of air through the severed supply line. 

2)The Trailer Supply valve will remain open, allowing a loss of air from the tractor until the 

reservoir pressure reaches 45 to 20 psi, or the operator closes the valve manually. 

3) The operator makes a foot valve application (green) to bring the tractor to a stop. That 

application air travelled to the tractor brakes, but “dead-ended”at the tractor protection valve 

preventing an air loss there. 


152

Supply Line Rupture (trailer intact) 

The supply line could rupture without the trailer breaking away: 

1) The glad hand simply was not properly connected and fell apart due to vibration. 

2) The hose itself ruptured due to chafing against a frame rail or for some other reason was 

punctured. 

3) The glad hand was torn lose from the supply line and lost on the road someplace. 

4) Any other irreparable failure you can think of. …next slide... 

153

When the supply line ruptures: 

Supply Line Rupture (continued) 

1) The emergency relay valve immediately dynamites the trailer brakes. (full on!) 

2) The trailer supply valve will close at between 45 and 20 psi (unless the operator closes it 

manually) 

3)The operator is looking for a place to pull over, since the trailer brakes are fully applied 

and the unit is coming to a stop real quick! 

4) The tractor brakes operate normally. (next slide) 

Left click to start animation. 

154

Supply Line Rupture (handling the situation) 

When the supply line has ruptured and the trailer has dynamited, the unit will come to an 

abrupt stop. To get back into service: 

1) If the glad hands simply came apart, reconnect them properly and recharge the trailer 

reservoir by opening the trailer supply valve. The trailer brakes will release and you are on 

your way. 

2) If the supply line glad hands are damaged beyond service, or if the supply line cannot be 

repaired, you can drain the trailer reservoir in order to release the trailer brakes. This will 

allow movement of the connected unit, but only for the purposes of relocating the vehicle to 

a safer location. (out of a tunnel, for example.) 

(1) 

(2) 

Left click for animation (1) and again for (2) 

155

Service Line Rupture 

The service line may rupture for the same reasons as the supply line may 

rupture. When the service line ruptures: 

1) Nothing will occur until a service brake application is made with either the 

foot valve or the hand valve. 

2) As soon as a service brake application is made, the trailer brakes will 

dynamite... 

…next slide... 

156

Service Line Rupture (continued) 

1) Because the air to apply the trailer brakes is taken from the supply line by the relay 

function of the tractor protection valve, this causes an immediate reduction in pressure 

in the supply line. 

2) The emergency relay valve “senses” this loss of pressure and interprets it as a severed 

supply line, and so, dynamites the trailer brakes. The one-way check valve in the 

emergency relay valve closes to protect the remaining air in the trailer reservoir. 

Air loss from service line 

upon service brake 

application 


157

Service Line Rupture (handling the situation) 

As with a supply line rupture, if the glad hands simply fell apart, they can be 

reconnected and the system recharged by opening the trailer supply valve. This will 

immediately release the trailer brakes and you’re on your way. 

If the service line is damaged beyond repair, you need not leave the cab in order to move 

the vehicle to a safe location. Just recharge the trailer reservoir, the trailer brakes will 

release and remain so until a service brake application is made again. 

If, for whatever reason, the above procedures cannot be carried out, simply drain the 

trailer reservoir and the trailer brakes will release. 


158

Loss of Main Reservoir Air 

In this case we’ve suffered a major failure in the main discharge line 

leading from the compressor to the wet tank. The low air warning devices 

activated by 60 psi. The one-way check valve between the wet and dry 

tanks prevented any loss of air from the dry tank. There is sufficient 

reservoir air left to make a brake application and bring the unit to a 

stop.The trailer brakes did not dynamite because the dry tank is still 

supplying reservoir air to the trailer reservoir. 


159

Crossed Lines (service and supply) 

Two things can happen if the supply and service lines are crossed. 

1. Trailer reservoir with no air: The law requires that a trailer reservoir hold its air for 

15 minutes only. After that, the reservoir may be considered to be empty, leaving the 

trailer with no brakes. No air, no brakes. 

2. Trailer reservoir with air: The trailer has been parked for only a short time (less than 

15 minutes) and there is air remaining in the reservoir. 

The following slides will depict both scenarios. Whatever the case, performing a check 

for proper trailer brake function before going into service is imperative. 

160

Supply & Service Lines Crossed (air in trailer reservoir) 

With air in the trailer reservoir and the lines crossed, reservoir pressure from the 

trailer supply valve will travel to the relay portion of the emergency relay valve on 

the trailer. 

The emergency relay valve will “try” to make a brake application on the trailer 

brakes in accordance with that delivered pressure. The result is, the trailer brakes 

will not release. This is a “dead give away” that the lines are crossed. 


161

Supply & Service Lines Crossed (no air in trailer reservoir) 

With no air in the trailer reservoir, the trailer brakes are disabled. Now with the 

lines crossed, the air from the trailer supply valve travels to the relay portion of the 

emergency relay valve. Again, the emergency relay valve “tries” to comply with the 

request for a brake application. However, the reservoir is empty, or contains very 

little air pressure, so the trailer brakes now will not apply. A tug test would confirm 

the lines were crossed since the trailer supply valve is open, the trailer is “charged,” 

but the brakes don’t work. 


162

Three Tests to Confirm Emergency Relay Valve Function 

Test 1 

Open the trailer supply valve to charge the trailer reservoir. 

Manually close the trailer supply valve at which time the trailer 

brakes should dynamite immediately. 


163


Test 2 

Set the tractor park brake. Open the trailer supply valve to charge the 

trailer reservoir. Step out of the cab and manually separate the supply 

line glad hands. The trailer brakes should dynamite immediately. (Air 

will flow out of the broken supply line leading from the tractor until the 

supply valve closes at between 45 and 20 psi.) 


164


Test 3 

Set the tractor park brake. Open the trailer supply valve to charge the 

trailer reservoir. Step out of the cab and manually separate the service 

line. Nothing should occur. Re-enter the cab and apply the hand or foot 

valve. The trailer brakes should dynamite immediately upon brake 

application. 


165


6. Single Circuit Tractor Trailer Systems: 

• Review 

166

Review Questions Section Six (Tractor-Trailer) 

1) Why is a tractor-trailer unit sometimes referred to as a “semi-trailer?” 


2) What is the purpose of a two-way check valve? 

3) Why should the glad hands be protected when not in use? 

4) How can an operator control the trailer brakes independently from the tractor brakes? 

5) Can the application pressure delivered to the tractor brakes exceed the pressure delivered to 

the trailer brakes? 

6) Can the application pressure to the trailer brakes exceed the application pressure delivered to 

the tractor brakes? 

7) What are three ways of testing the emergency function of the emergency relay valve? 

1) because 6) 7) Yes. close This the some supply can be of valve; advantages the weight break at of the times the supply trailer of poor line; is carried road break conditions the by the service tractor line 

2) 3) 4) 5) No. by delivers to and keep use The make of them higher two-way a service hand clean pressure check valve and brake free derived valve application: of will debris from not and two trailer allow damage sources should this dynamite in all 

cases 

167

Review Questions Section Six (Tractor-Trailer) 

8) What is the main purpose of the trailer supply valve? 


9) What valve supplies air to the trailer reservoir? 

10) What will occur if the supply line ruptures? 

11) What will occur if the service line ruptures? 

12) What will occur if a service brake application is made when the service line is ruptured? 

13) What are the three functions of the emergency relay valve? 

14) If the foot and hand valve were applied at the same time, can the application pressure be 

greater than the reservoir pressure? 

11) 9) Nothing. 13) the 14) dynamites emergency No. Application 

Until a trailer relay service brakes; valve: pressure it’s quicker the can last never 

application application valve exceed 

is made, the and air that 

at flows which release through time of trailer 

8) 12) 10) provides before the trailer arriving a means will at dynamite 

of the “charging” trailer immediately 

the trailer reservoir 

the trailer brakes; which 

will one-way is present 

dynamite. check the valve reservoirs. prevents loss of trailer reservoir air 

168


7. Dual Circuit Systems: 

169

Section Seven 

Dual Air 

The Canadian Motor Vehicle Safety Standard (CMVSS) was introduced in 1975. 

•All vehicles must be equipped with front brakes 

•All vehicles will incorporate a “dual air brake system.” 

By the end of this session you will be familiar with: 

•The reasons for bringing in the CMVSS 

•The differences between single circuit and dual circuit systems. 

•The extra benefits of incorporating a dual circuit air brake 

system. 

•Dual circuit systems in semi-trailer applications. 

170

The Purpose of Dual Air Systems 

Dual circuit air brake systems provide the following advantages: 

1) provide a shorter stopping distance. 

2) reduce brake failures due to mechanical faults. 

3) delays the automatic application of the spring brakes in an emergency, 

keeping control of the vehicle in the hands of the operator, longer. 

4) allows the operation of one part of the system even though the other 

part has failed. 

171

Identifying Primary and Secondary Systems 

The Compressor, Governor, and Supply Reservoir appear in blue. The 

safety valve is above the wet tank. 

Air from the supply tank, flows into TWO new reservoirs. The GREEN 

reservoir will be supplying air to the REAR brakes. The RED reservoir 

will be supplying air to the FRONT brakes. 

The REAR brakes will be referred to as the PRIMARY brakes. 

The FRONT brakes will be referred to as the SECONDARY brakes. 

There is a very good reason for making this distinction… next slide... 

172

Identifying Primary and Secondary Circuits (continued) 

The arrows indicate forces applied during braking... 

On short wheel base vehicles, such as cars motorbikes and small 

trucks and vans, when a brake application is made, there is a 

transfer of weight onto the front wheels. (Most of these vehicles will 

be equipped with disc brakes at the the front and drum brakes at 

the rear.) This further indicates that the majority of braking is 

accomplished by the front brakes. 

173

Identifying Primary and Secondary Systems (continued) 

On a long wheel base vehicle such as a semi-trailer or other larger 

commercial vehicle, a bus for instance, there is no such transfer of 

weight onto the front axle. During braking, even heavy braking, the 

weight of the vehicle remains over the drive axles and the trailer axles 

(if equipped.) Therefore, the majority of braking effort is 

concentrated at the rear axle or axles. …next slide... 

174

Primary and Secondary Systems (continued) 

Short wheel 

base vehicle 

Long 

wheel base 

vehicle 

Secondary Brakes 

Primary Brakes 

Secondary Brakes 

Primary Brakes 

Trailer Brakes 

175

Dual Circuit 

Foot Valve 

The Simple Dual Circuit System 

In this diagram, the blue lines indicate “supply air”, the red lines 

indicate “secondary” or front brake air and the green lines indicate 

“primary” or rear brake air. A dual circuit foot valve has been 

installed. This foot valve works in exactly the same way as a single 

circuit foot valve. However, the dual circuit foot valve is split 

internally into two parts. The upper part delivers application air to 

the front brakes, while the lower part delivers application air to the 

rear brakes. 

Compressor 

Secondary 

Reservoir 

Supply 

Reservoir 

Primary 

Reservoir 


176

The Simple Dual Circuit System 

One-way check valves have been added at the inlets to the primary and 

secondary reservoirs. Should an air loss occur at the supply reservoir these 

one-way check valves will prevent the air from the primary and secondary 

reservoirs from back-flowing into the supply reservoir. 


177

Secondary Air Loss in a Simple Dual Circuit System 

In this example, an air loss will occur in the secondary system. The one-way check valve at the 

inlet of the primary reservoir will close, preserving the primary air. The low air warning devices 

will activate due to the air loss (either reservoir will trigger them.) As the pressure drops, the 

governor will load the compressor, but that air will simply be pumped to atmosphere due to the 

faulty secondary system. 


178

Secondary Air Loss (Foot Valve Application) 

The operator now makes a foot valve application. Because there is no reservoir air in the secondary 

tank, the front brakes will not work. The rear brakes, however, are still functional because the oneway 

check valve at the primary reservoir closed. Since the rear brakes are the primary brakes, the 

vehicle can be brought to a safe stop with little difficulty. Of course, continuing to operate the 

vehicle under these circumstances would be unsafe, if not irresponsible. 


179

Primary Air Loss in a Simple Dual Circuit System 

This time the air loss occurred in the primary system. The one-way check valve at the inlet to the 

secondary reservoir will close to protect that air. The low air warning devices will activate by 60 psi., 

and the governor will load the compressor at 85 psi or better. Again, air from the compressor is being 

pumped to atmosphere. The front brakes are the only brakes working. Since this is a long wheel base 

vehicle, where most of the braking is accomplished by the rear brakes, we are in serious trouble! We’ll 

see how to fix this undesirable situation in a few minutes. 


180

Adding the Park Brake (spring brake) 

A two-way check valve has been added just downstream from the primary and secondary reservoirs. 

During normal operation, air from both reservoirs flows through this two-way check valve, in 

approximately equal amounts. This air is referred to as “blended air.” It then flows through the park 

brake control valve (when open) and on to the spring brakes, holding them in the released position. 


181

Secondary Air Loss and Park Brake Function 

When a secondary air loss occurs, the two-way check valve will move towards the low pressure side 

(secondary reservoir.) This prevents air loss from the primary reservoir into the secondary reservoir. 

Because the primary reservoir still contains air pressure, and the two-way check valve has covered up 

the secondary reservoir, primary air only will continue to hold the spring brakes in the released 

position. This delays the automatic application of the park brake, allowing the operator more time to 

choose a safe place to pull over. Also, there is no rear wheel lock-up as occurs in a single circuit system 

when air loss occurs. 


182

Primary Air Loss and Park Brake Function 

In a primary air loss situation, the subsequent events are similar to a secondary air loss. 

The two-way check valve moves toward the low pressure side (primary reservoir.) The 

one-way check valve at the inlet to the secondary reservoir closes. The park brakes 

remain released, held so by secondary air only. Now, however, we’ve lost the primary 

brakes so stopping the vehicle becomes much more difficult. The operator would need to 

manually apply the park brake, which could result in rear wheel lock-up. This is one of 

the events we were trying to prevent by adding dual air. 


183

The Potential Energy in the Spring Brake 

The compressed spring inside this park brake chamber contains Potential Energy. This is energy that 

is stored, and can be used at a later time. When the primary air system fails, we can use the potential 

energy contained in the spring brake to apply and modulate the rear service brakes without the use of 

primary system air. The spring brakes, in fact, can take the place of the rear service brakes. 


184

Enter the Modulator Valve 

The modulator valve is sometimes referred to as an “inversion” valve. Inverse simply 

means opposite from the regular way a valve might function. For example, when the 

operator presses down on the foot valve, air pressure is delivered. If the operator could 

“press down” on the modulator valve, it would release air pressure: work inversely. 

The modulator valve is pictured here as No. 27 in the 

diagram.The next few slides will walk us through the 

operation of the modulator valve, which, along with the 

spring brakes, are essential to dual air operation. 

185

The Modulator Valve (continued) 

The modulator valve is never required to fulfil its function until the system suffers a primary air loss. 

To understand its function in the system, let’s first look at normal operation. When the system is 

charged and operating normally, primary air pressure is present at the modulator valve as indicated 

by the green arrow. Air pressure from the two-way check valve (blended air) flows through modulator 

valve and on to the spring brakes, holding them released (orange arrow.) Application air from the 

front brakes arrives at the point indicated by the magenta arrow. 

. 

186

The Modulator Valve (secondary air loss) 

Foot Valve Application 

When a secondary air loss occurs, the front brakes will not function. As we now know, 

the rear brakes are the primary brakes (most of the braking effort is accomplished by 

the rear brakes) so the vehicle can be brought to a safe stop at the operator’s 

discretion. The modulator valve plays no role in this scenario because there is pressure 

in the primary reservoir. The spring brakes remain released, held so by primary air 

pressure only, as opposed to blended air. 


187

The Modulator Valve (primary air loss) 

When the primary system is affected by an air loss, the rear service brakes will not work. 

Remembering that the spring brakes contain potential energy and the ability to take the 

place of the rear service brakes, let’s take a look at how the modulator valve facilitates 

this function. First, we will open the drain cock on the primary reservoir to simulate a loss 

in the primary system. The one-way check valve at the secondary reservoir closes. The 

two-way check valve moved down to protect the secondary reservoir. Secondary air only 

now holds the park brake released. …next slide... 


188


When the primary system loses pressure, the modulator valve “responds” to this 

condition because the pressure normally present at the modulator valve (arrow) is 

now gone. Also, the primary air supply to the foot valve is lost, rendering the rear 

service brakes useless.(arrow) …next slide... 


189


Foot Valve Application 

With rear (primary) service brakes out of action, when the operator makes a foot 

valve application, secondary application air travels not only to the front brakes, but 

to the modulator valve as well. (circle.) No air is delivered to the rear service 

brakes, because the primary reservoir is empty. …next slide... 


190


Foot Valve Application (continued) 

The modulator valve “senses” the foot application, and releases air from the spring 

brakes, but only as much as required by the foot valve application. The modulator 

valve is “modulating,” or varying the release of air from the spring brake chambers 

so that the rear brakes will apply at roughly the same pressure as the front brakes. 


191


7. Dual Circuit Systems: 

• Review 

192

Dual Air System Summary 

1) The purpose of dual air is to shorten stopping distance, reduce brake failures, and split 

the system so that if one side fails, the other works 

2) When secondary air is lost, the front brakes are sacrifice. They are less important because 

most of the braking is accomplished by the rear brakes. 

3) When primary air is lost, the spring brakes are employed as replacements for the nonfunctioning 

rear service brakes. The modulator valve “senses” the primary air loss and 

begins to “modulate,” or vary, the brake pressure of the spring brakes in accordance with 

the front service brake application. 

4)Whenever a system failure occurs, the operator must pull over to the side of the road and 

stop the unit. Even if the compressor is pumping, it is pumping out to atmosphere, so the 

system cannot be replenished. 

5) Since the air brake system is split into two parts, each part (primary or secondary) is 

capable of keeping the spring brakes released. Remember the two-way check valve? This 

delays the automatic application of the park brake allowing the operator more time to select 

a place to pull over and stop. 

6) Under normal operation, the air from both primary and secondary systems “blend” at the 

two-way check valve. One-way check valves are found at the inlet of the primary and 

secondary reservoirs. …2 

193

Dual Air System Summary (continued) 

7) The primary role of the spring brake is as a parking brake. 

8) When primary system air is lost, the modulator valve and the spring brakes work 

together to provide an emergency braking system for the rear axle. The modulator valve, 

at the command of the foot valve, will release an amount of air roughly equal to that being 

applied at the front brakes by secondary air.( A good way to remember this concept is to 

recall that the modulator valve is sometimes referred to as an “inversion valve.”- meaning 

it works inversely or opposite to normal function. Pressing down on the foot valve applies 

air to the brakes, where “pressing down” on the modulator valve will cause air to be 

released.) 

9) The reservoir air gauge in a dual air equipped vehicle, will usually have two needles. 

One needle indicates primary air (usually green), the other needle is red or yellow 

(secondary air). When fanning air off during a pre-trip inspection, both needles will drop 

at approximately the same rate. When pressure reaches between 45 and 20 psi., the park 

brake should apply automatically 

10) When fanning air off to observe governor cut-in pressure (85-105 to max.135), it 

doesn’t matter which system arrives at that pressure first. Either one should place the 

compressor into it’s pumping stage. 

10) Some vehicles will have a separate gauge for each system. There could be 2, 3, 4, or 

more depending on the system being used. 

194


8. Dual Circuit Systems ‐ Trailers: 

195

Combination Truck and Trailer with Spring Brakes 

Blended air from the two-way check valve (22) flows to the trailer supply valve (24). From there, to the 

tractor protection valve (23). The lower line (20) carries reservoir air (supply line), the upper line (20) 

is the service line carrying application air as needed. The trailer is coupled in the usual way with 

standard glad hand couplers. The foot valve (6) and hand valve (21) are as they were in the single 

circuit system studied earlier. ...next slide... 

196

Combination Truck and Trailer with Spring Brakes 

(continued) 

The tractor incorporates two relay valves (14). One relay valve is piped into the primary service 

brakes, the other is piped into the park brake system (allows quicker application and release of the 

park brake.) The modulator valve (27) is situated in the centre of the diagram. There is a front brake 

limiting valve (17) in the centre of the front axle. …next slide... 

197

Combination Truck and Trailer with Spring Brakes (continued) 

The tractor and trailer of this unit are equipped with spring brakes. When the trailer supply valve is 

opened, air flows through the tractor protection valve and on to the “spring brake valve” on the 

trailer. Note: there is no emergency relay valve. When a brake application is made, air flows to the 

tractor protection valve, is relayed to the service line, arriving at the trailer relay valve which delivers 

the required application to the trailer brakes from the trailer reservoirs. 


198

SYSTEM 

PARK 

TRAILER 

SUPPLY 

TRACTOR 

PARK 

This system uses three valves, (24) trailer supply 

valve, (19) system park (applies all park brakes), 

and (19) tractor park, which is useful for 

securing the tractor-trailer when “setting up” the 

trailer brakes. …next slide... 

199

Combination Truck and Trailer (Supply Line Rupture) 

In an emergency, or if the supply line ruptured, the trailer will dynamite immediately. The 

difference between dual air and single circuit is: spring brake equipped trailers exhaust all the 

air from the trailer spring brakes which apply the brakes. They do not use air pressure! 


200

Combination Truck and Trailer (Supply Line Rupture continued) 

With the supply line ruptured, the trailer dynamites immediately. There is no air loss from the 

trailer reservoirs, as the spring brake pressure protection valve closes. The trailer spring 

brakes fully apply. The tractor protection valve prevents air loss from the tractor. The tractor 

brakes work normally. 

201

Combination Truck and Trailer (Service Line Rupture) 

With only the service line ruptured, like the old single circuit system, nothing occurs until a 

service brake application is made. Then, the trailer dynamites as previously described. If the 

trouble can be repaired, re-charging the trailer will release the spring brakes and the unit is 

back in business. 


202

Combination Truck and Trailer (mixed units) 

A dual circuit tractor is able to pull a single circuit trailer, and visa versa. However, a spring 

brake equipped trailer, once dynamited and the trouble is not repairable, must have its spring 

brakes “wound off.” There is no emergency release reservoir. …next slide... 

203

Combination Truck and Trailer (Secondary Circuit Failure) 

A failure in the secondary circuit would render the tractor front brakes inoperative. The 

tractor rear brakes (primary) continue to function normally. The trailer does NOT dynamite 

because the intact primary circuit now supplies air to the trailer reservoir. 

Primary circuit intact and 

supplying air to trailer. 

Service brakes work 

normally. 

Two-way check valve moves upward 

to protect the primary circuit. 

204

Combination Truck and Trailer (Primary Failure) 

In this example, the primary system has failed. As before with just a truck or bus, the modulator 

valve has sprung into action. Although the rear service brakes cannot be supplied with air, the 

spring brakes have applied in accordance with the foot valve application made by the operator. The 

trailer has not dynamited because the secondary system is supplying air to the trailer reservoir. 

Secondary system 

supplying air to trailer. 

Brakes work normally. 

Two-way check valve protects the secondary 

from the primary circuit. 

205


8. Dual Circuit Systems ‐ Trailers: 

• Review 

206

Section Review - Dual Air - Tractor Trailer 

1)What is the principal of a dual circuit air brake system? 

2)What valve is used to protect the primary circuit from the secondary circuit? 

3) In a dual circuit system, if the secondary system failed, which brakes would still work? 

4) What are two functions of the spring brakes in a dual circuit system? 

5) If the primary circuit in a dual air system failed, which brakes on the vehicle would be 

available for braking? 

6) If the trailer were to break away on a dual circuit system, what would apply the brakes on 

the trailer: a) air pressure or b) spring pressure? 

7)What is blended air and where would it be found? 

8) At what pressure does an automatic type trailer supply valve close in the event the trailer 

breaks away? 

9) What is the purpose of the modulator valve? 

10) What valves are found at the inlet of the primary and secondary reservoirs? 

9) 10) 3) 4) 6) 7) 8) 2) 5) to between air rear first one-way 

1) b) front two-way modulate as brakes a parking 

if spring from 

one and 45 both check 

circuit pressure check rear: (vary) and only primary valves 20 

fails, valve the brake, spring psi. spring and secondly brake as a back up system in the 

event of primary circuit 

the 

failure 

other brakes secondary pressure works: are circuits 

redundancy modulated when mixing the to primary take just 

circuit 

the 

after place leaving fails 

of the rear their service respective brakes. reservoirs 

207


9. Off Highway Units: 

208

T-75 Off Highway System - Single Unit (mining equipment etc.) 

As complicated as it looks, all that really has changed, is another reservoir has been added. The 

brown reservoir supplies air to the front brakes only. The two rear axles are now separated 

into primary and secondary systems. Essentially, each axle has its own system. …next slide... 

209

T-75 Off-Highway System (continued) 

When a brake application is made, the foot valve sends application air to the front brakes from 

the brown reservoir, to the first rear axle from the red reservoir (secondary) and to the back 

axle from the green reservoir (primary). By this method, all axles are isolated from each other, 

and a failure of any one would not affect the other two. Of course any failure of any of the 

systems would, as always, require the operator to stop immediately and fix the trouble before 

continuing work. 

210

L-75 Off Highway Combination with Schedule “X” Trailer 

Scary? Not really. The tractor is similar to the T-75 truck we just looked at. The components that have been added 

include:(31) a low pressure governor, (32) a dump valve and (33) a relay type tractor protection valve. The trailer (schedule 

x) is not equipped with spring brakes. Each axle of the tractor has its own system, each protected from the other by oneway 

and two-way check valves, as we have seen. The trailer has two air systems as well. If one failed, the other will 

continue to work. Hand valve function and emergency trailer functions are the same. Break-away or ruptured supply line 

will dynamite the trailer. …next slide... 

Low press.Gov. 

Dump Valve 

Trailer supply 

Supply line gauge 

211

L-75 Off Highway Combination with Schedule “X” Trailer (continued) 

When air pressure in the main reservoir system is above the setting of the low pressure governor (approx. 

50 psi) the exhaust ports are held closed and the trailer emergency line can be charged normally. If main 

reservoir pressure drops below the governor setting, the governor shuts off and exhausts air from the 

control port of the dump valve. The exhaust ports on the dump valve open, venting the emergency (supply) 

line to the trailer. This will cause the trailer brakes to “dynamite” and the trailer air supply dash valve to 

close, shutting off further air supply to the trailer. 

Low press. Gov. 

Dump Valve 

Trailer supply 

Supply line gauge 

212

Schedule SX Trailer (spring equipped with on trailer control valve) 

This trailer is equipped with spring brakes. It is an off-road trailer common to the logging 

industry. Note the extra reservoir (2) that allows for the release of the spring brakes even if 

the trailer is not hooked up. This allows movement of the trailer so it can be loaded onto 

the tractor for transportation back to where the active logging is taking place. 

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10. Summary, Questions, Wrap‐Up: 

214

Thank you for viewing this Air Brake Review! 

This presentation is based on the B.C. Air Brake Manual and includes tractor- 

trailer systems and some off-highway equipment that is not relevant to transit 

employees. It includes this material so as to provide a complete 

refresher/reference source as possible. It should not, however, be considered to be 

the official source for this information. The Government of the Province of 

British Columbia produces several publications that should be referred to in 

combination with this presentation. In any case, the Training Department of 

Coast Mountain Bus Co, BC Transit, or any of the Company’s s employees 

involved in the production of this presentation will be held blameless in any 

accident, injury or harm arising out of its use. 

If you have suggestions, comments or criticisms please contact: 

BC Transit - Safety, Training & Security Department 

(250) 385-2551 

215

BC Transit â Air Brake Course

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