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

Using the Brake System
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 octagonal-shaped
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 to the
trailer, and (3) protect the tractor air supply in case of separation
of the air lines connecting the tractor to the trailer.
Hydraulic Brakes
Introduction
Operating Instructions
WARNING! Do not operate the vehicle in the
event of a malfunction in any air or hydraulic circuit.
Such a malfunction may prevent the brake
system from operating properly, and could
result in an injury accident. The vehicle should
not be operated until the system is repaired and
both braking circuits, including all pneumatic,
hydraulic, and mechanical components are
working properly.
The operation of the vehicle's braking system is based on the
principle of hydraulics. Hydraulic action begins when force is
applied to the brake pedal. This force creates hydraulic pressure
in the master cylinder and is amplified with assistance of
a power booster. The supplemental boost in force is developed
when pressurized power steering fluid from the steering
pump presses on the master cylinder piston. As a safety precaution,
the pressurized fluid from the master cylinder has
two mutually independent circuits. The primary circuit supplies
the front wheels while the secondary circuit supplies the
rear wheels. The displaced fluid from the master cylinder
travels through brake pipes terminating at the wheel cylin-
– 88 – Y53-6008 Medium Duty (R10/06)