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

Operating Instructions
Front Brakes — Truck
The following applies to full truck configurations:
If air pressure is lost in the front circuit, the vehicle front
brakes will not operate. Check valves isolate the rear circuit,
so the rear service brakes and parking brakes are unaffected.
The spring brakes will not automatically apply as long
as rear service pressure is maintained.
WARNING! If the remaining system pressure
drops below 60 psi (414 kPa) due to repeated
brake applications, the spring brakes will automatically
apply—stopping the vehicle abruptly,
potentially causing serious injury. Observe the
warning light (BRAKE AIR) in the panel. If it
comes on, do not continue to drive the vehicle
until it has been properly repaired or serviced.
Rear Brakes — Truck
The following applies to full truck configurations:
If air is lost in the rear brake circuit, check valves isolate the
front circuit, and front brake operation is unaffected. Because
this is a dual air system, spring brakes will not automatically
apply as long as pressure is maintained in the front circuit.
Using the Brake System
When the brake pedal is depressed, the modulating valve
detects the reduction or absence of rear service delivery pressure,
and exhausts air from the spring chambers, applying the
spring brakes in proportion to the front brake application.
When releasing the brake pedal, the modulating valve delivers
front service air to the spring brake chambers, releasing
the spring brakes in proportion to the front brake application.
Thus, if there is a rear service failure, the remaining front axle
braking is supplemented by modulated spring brake action
on the rear axle.
WARNING! Efficiency of emergency braking
depends on the amount of air available in the
front circuit reservoir, which is sufficient for a
limited number of brake applications and
releases.
• REMEMBER: when the remaining system pressure
drops below 60 psi (414 kPa), the spring brake is
automatically applied, stopping the vehicle abruptly,
potentially causing serious injury. Observe the warning
light (BRAKE AIR) in the panel, it will come on
before the spring brakes are applied. If it comes on,
do not continue to drive the vehicle until it has been
properly repaired or serviced.
Medium Duty (R10/06) Y53-6008 – 79 –