Peterbilt Medium Duty Truck Owner's Manual - Peterbilt Motors

USING THE BRAKE SYSTEM
• Modulates spring brake application in proportion to
front service application in the event of a rear service
failure.
• Prevents compounding of service and spring applications.
Relay valve (full truck): speeds up the application of the
rear service brake. It also incorporates a quick–release
feature.
Brake Proportioning (BP-R1) valve (tractor): this valve
delivers air to the tractor's service brake chambers. It
automatically proportions the drive axle brake app
lication pressure when driving bobtail (without a trailer
connected). Proportioning the brakes allows full use of the
steer axle brakes and reduces lockup of the drive axle
brakes. The bobtail brake proportioning system is automatically
turned off when a trailer is connected.
Quick–Release Valve: speeds the release of air from the
brake chambers. When air is released, the air in the brake
chambers is exhausted at the quick-release valve, rather
than exhausting back through the treadle valve.
STARTING AND OPERATING THE VEHICLE
Single Check Valve: allows air flow in one direction only.
Parking Brake Valve: yellow diamond-shaped knob. It
controls the application and release of the parking
(spring) brakes of truck or tractor-trailer combinations or of
the tractor alone. If the air system is being charged from
zero pressure, the parking brake valve will not hold in the
release position until the system pressure exceeds 60 psi
(414 kPa), which is the pressure required to override the
load of this valve's plunger return spring.
Trailer Supply Valve (tractor): The Red octagonalshaped
knob protects the tractor system; it functions in
conjunction with the parking brake valve (yellow). The
trailer supply valve is responsible for synchronizing the
tractor and trailer parking and emergency brakes. If the air
system is being charged from zero pressure, the trailer
supply valve will not hold in the applied position until the
system pressure exceeds 50 psi (345 kPa). It automatically
pops out and exhausts air if supply air pressure
drops below 60 psi (414 kPa).
Tractor Protection Valve: The functions of this valve are
to (1) receive all pneumatic signals pertinent to the operation
of the trailer brake system, (2) transmit these signals
– 120 – Y53-6027 Medium Duty (R05/09)