Medium Duty Operator's Manual - Peterbilt Motors Company

USING THE BRAKE SYSTEM
(2) Trailer Air Supply
Control (Red)
(1) Parking Brake Control
(Yellow)
STARTING AND OPERATING THE VEHICLE
To release the trailer brakes ONLY:
• Push IN the Red knob on the dash. The truck or tractor
will remain parked.
Combination (Tractor/Trailer) Parking Brake Control Valves
The parking brakes act on the rear wheels only. They are
spring-applied, with air pressure used to release them.
Release air is supplied by both the front and rear circuit
reservoirs through a double check valve.
To release the truck or tractor parking brakes ONLY:
02394
• Push IN the Yellow knob on the dash. Your trailer will
remain parked.
Below 60 psi (414 kPa) the Yellow parking brake valve
remains OUT (ON position). If air pressure is not
restored above 60 psi (414 kPa), the knob will automatically
return to the OUT position if you attempt to
push it in. See page 41 for more information.
To release the full combination of brakes:
• Push IN BOTH knobs on the dash.
• In the event that air pressure is reduced below a safe
level: the low air warning light will come on first; if air
pressure continues to drop, the parking brake valve
will pop OUT, automatically applying the spring
brakes.
CAUTION: Do not try to put the vehicle in
motion before pressure in the system
reaches 100 psi (689 kPa) because the
wheels are locked by the spring brake action.
•Unnecessary stress and possible brake malfunction
could occur if the vehicle is forced to
move before the air system reaches 100 psi
(689 kPa).
•Below 60 psi (414 kPa) the parking brake valve
button remains in the applied (ON) position. If
air pressure is not restored, the button will
automatically return to its initial position.
– 104 – Y53-6008A Medium Duty (R05/09)