Peterbilt Medium Duty Trucks Operator's Manual after 1-07

Using the Brake System
Brakes — Tractor/Trailer
The following applies to tractor/trailer configurations:
If pressure is lost in the tractor front or rear circuit, the “check”
valves isolate the unaffected circuit, allowing this circuit to
continue normal operation. The trailer brakes are still functional.
• If air pressure is lost in the trailer supply/park circuit, and
the pressure drops below 60 psi (414 kPa), the trailer
spring brakes are automatically applied, and the tractor
air pressure circuits are unaffected.
• If air pressure is lost in the trailer brake service circuit,
and the pressure in the tractor front and rear circuits
drops below 60 psi (414 kPa), the tractor and trailer
spring brakes are automatically applied.
Emergency Braking
WARNING! Unless you have an anti-lock braking
system, always avoid completely depressing
the service brake pedal, if possible, even
during emergency braking. Depressing the
brake pedal too aggressively can cause the
wheels to lock, which can lead to an uncontrolled
skid and can result in an accident.
Operating Instructions
For Non-ABS Vehicles: To stop your vehicle in an emergency,
vary the service brake application pressure to provide
maximum braking force without locking the wheels. Use
engine compression to assist the service brakes by not
depressing the clutch pedal until the engine reaches idle
speed.
Overheated Brakes
Under normal braking conditions, the energy generated will
bring the internal brake drum temperature to about 500°F
(260°C). This is well within the safe zone: the maximum safe
temperature of lining for drum type brakes is usually about
800°F (427°C).
But if service brakes are used for emergency braking, used
improperly, or for prolonged periods, internal brake drum
temperatures may exceed 800°F (427°C). Such brake overheating
may be detected by a burning smell or smoke coming
from a drum. If this occurs, you should immediately stop and
check for cracked brake drums or lining fires. If neither exists,
get back behind the wheel and resume a slow speed as soon
as possible to cool the brakes. If the vehicle was to remain
stopped, the heat transfer could destroy the linings and distort
the brake drum.
– 80 – Y53-6008 Medium Duty (R10/06)