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

Operating Instructions
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 quick-release
valve.
Using the Brake System
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.
Medium Duty (R10/06) Y53-6008 – 73 –