SERVICE MANUAL
PARKING BRAKE APPLICATION
To apply the spring (parking) brake, the operator of the
vehicle positions the control valve in the applied position
which exhausts air pressure from the spring brake
loaded chamber (Fig. 4). Upon reduction of air
pressure in the chambers, the parking brake springs will
apply the rear brakes through the spring brake pushrod
and service brake pushrod assemblies which are in turn
connected to brake slack adjusters or brake wedge
assemblies.
Fig. 4 - Parking Brake Application
To release the parking brake, the operator positions the
control valve to the released position which actually
applies air in the spring brake system, thereby loading
the chambers, and air pressure overcomes the heavy
springs
forcing
the
spring
brake
pushrod
and
diaphragms to released position or normal driving
position (Fig. 2).
Upon loss of air pressure or reduction of air pressure in
the vehicle air system, the spring (parking) brakes will
automatically apply, providing an emergency brake.
Refer to Air Brake System (FMVSS-121) section that
applies to your vehicle for complete detailed instructions
pertaining to the operation of the "air system" in the
event loss of air is ever encountered.
MANUAL RELEASE OF SPRING BRAKE
To release the spring brake (manual release)
in the event there is loss of air and the vehicle must be
towed or whenever the spring brake chamber assembly
is removed from the vehicle, the spring brake must be
manually released or "caged" (Fig. 5).
Fig. 5 - Spring Brake In Manual RIoas4ed Position
To manually release the spring brake chamber you are
actually compressing the spring to relieve any tension on
the pushrod assembly.
WARNING
Before manually releasing the spring brakes,
be sure to block the wheels so that the
vehicle cannot move when the brakes are
released. For towing, make sure the vehicle
is connected or secured to tow vehicle
before releasing the spring brakes.
The spring brake chamber can be manually released
(caged) in two manners.
If the vehicle has 586 kPa (85 psi) of air or more,
manually
releasing
the
spring
brakes
can
be
accomplished easier with air in parking brake system if
the breather cap is removed and the release bolt backed
off counterclockwise as follows:
Type 24 and 30 chambers 57 to 64 rwm (2-1/4 to 2-1/2")
Type 36 chambers 70 to 76 mm (2-3/4 to 3").
CTS-4101W - Page 4
