Medium Duty Operator's Manual - Peterbilt Motors Company

USING THE BRAKE SYSTEM
Brake Operation
WARNING! Do not drive through water deep
enough to wet brake components, as it may
cause the brakes to work less efficiently than
normal. The vehicle's stopping distance may
be longer than expected, and the vehicle may
pull to the left or right when brakes are
applied, which could contribute to an accident.
If you think your brakes have become wet, check the rear
and both sides of the vehicle for clear traffic, then gently
apply the brakes, releasing and gently reapplying until the
brakes dry out—restoring normal operation. Always check
your brakes after driving through deep water to help
reduce the possibility of personal injury or an accident.
Front Brake System
When the brake pedal is depressed, the front circuit portion
of the treadle valve delivers air from the front service
reservoir to the front axle brake chambers via a quickrelease
valve.
STARTING AND OPERATING THE VEHICLE
Simultaneously (on full truck configurations), air is also
supplied to the modulating valve control port. In the event
of a rear service circuit failure, the modulating valve will
exhaust air from the spring brake chambers, applying the
spring brakes in proportion to the front circuit application.
Rear Brake System
When the brake pedal is depressed, the rear circuit portion
of the treadle valve delivers air from the rear service
reservoir to the service brake relay valve control port. The
relay valve then delivers air directly from the rear service
reservoir to the rear brake chambers in proportion to the
treadle pressure.
On tractors without ABS, the relay valve is part of the BP-
R1 brake valve, which automatically proportions the drive
axle brake application pressure when driving bobtail (without
a trailer connected). The proportioning BP-R1 brake
valve allows full use of the steer axle (front) brakes and
reduces the chance that the drive axle brakes will lockup.
The bobtail brake proportioning system is automatically
turned off when a trailer is connected.
– 102 – Y53-6008A Medium Duty (R05/09)