Medium Duty Operator's Manual - Peterbilt Motors Company

USING THE BRAKE SYSTEM
STARTING AND OPERATING THE VEHICLE
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.
NORMAL OPERATING
POSITION
PARK TRAILER WITH
TRACTOR RELEASED OR
BOBTAIL TRACTOR
SYSTEM PARK OR
CHARGE TRAILER
WITH TRACTOR PARKED
TRAILER
AIR
SUPPLY
PARKING
BRAKE
TRAILER
AIR
SUPPLY
PARKING
BRAKE
TRAILER
AIR
SUPPLY
PARKING
BRAKE
RED
YELLOW
RED
YELLOW
RED
YELLOW
02393
Trailer Park or Emergency Brake Application Only
If you ever have a failure or disconnect the air supply hose
to the trailer, the trailer parking brakes will set. The Red
knob will automatically pop OUT and seal off the tractor
air reservoirs to protect the tractor air system pressure.
To apply the trailer brakes only:
Pull OUT the Red knob. This will exhaust air from the
trailer supply line, causing the tractor protection valve to
close and the trailer spring brakes to apply. The trailer is
now in “emergency” or “park.” This mode would be used to
uncouple from the trailer and during bobtail operation
(running without a trailer connected).
– 106 – Y53-6008A Medium Duty (R05/09)