Medium Duty Operator's Manual - Peterbilt Motors Company

USING THE BRAKE SYSTEM
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
STARTING AND OPERATING THE VEHICLE
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.
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.
– 108 – Y53-6008A Medium Duty (R05/09)