GMC 4000 Series Service Manual page 235

Sec. 5B
Page 228
AIR BRAKES
control valve and the open exhaust port of the foot
control valve.
When the DD3 brake actuator lock port is
vented the roller spring forces the rollers against
the ramp on the collar to engage them with the
push plate shaft.
When the control port of the inversion valve
is vented, the inversion valve piston moves for­
ward and opens the inlet exhaust valve. This opens
the line from the parking reservoir and allows air
pressure to flow into the parking port of the DD3
brake actuator. Full parking reservoir pressure is
delivered and a parking brake application results.
With the rollers against the shaft, the shaft can
move forward but is locked so that it cannot return
for release. To release parking brake application
it is necessary to have full pressure in the air
system , "push in" on push-pull control valve (make
sure it stays "in") and make a full 100 psi service
brake application.
EMERGENCY OPERATION
The vehicle is equipped with an air pressure
gauge (in the instrument cluster) and (as optional
equipment) a low air pressure buzzer. When the
operator is aware of a lo ss of air pressure as in­
dicated by the gauge or the buzzer, and the service
brakes will not stop the vehicle, a manual em er­
gency application of the parking brake should be
made by pulling out the handle of the push-pull
control valve. Brakes then operate as described
under the heading of "Parking Operation."
In the event the operator does not respond to
the warning system or if the system fails to func­
tion, or if the lo ss of air pressure is too sudden
for action, and if the air pressure in the parking
reservoir falls below 40 psi, the push-pull control
valve will automatically "pop" out, causing p res­
sure in the DD3 actuator lock port to exhaust,
which will result in an automatic parking (em er­
gency) brake application.
DD3 BRAKE ACTUATOR OPERATION
(Refer to Figure 19)
NORMAL RUNNING
Through the push-pull control valve and an
inversion valve, air enters the actuator locking
port and exerts pressure on the locking piston
grommet. The resultant force moves the locking
piston forward against the rollers and roller
spring. The beveled, or ramp, end of the piston
will pick up and hold the rollers away from the
shaft. As long as air pressure remains against
the locking piston grommet and the rollers are not
in contact with the shaft, normal service brake
applications will permit the shaft to move freely,
back and forth, past the locking mechanism. When
G M C S E R V I C E M A N U A L
a normal service brake application is made, air
enters the actuator service port and applies p res­
sure against the service diaphragm. The diaphragm
moves the push plate and shaft out, applying the
brakes. Upon the release of the service applica­
tion, the brakes are released.
PARKING
port and air is applied against the parking dia­
phragm through the parking diaphragm port. When
air is exhausted from the lock piston, the roller
spring forces the rollers against the collar and
shaft. Air entering the parking port exerts force
on the parking diaphragm. The diaphragm moves
the push plate and shaft out, applying the brakes.
With no air on the lock piston, the shaft becomes
mechanically locked in the applied position as the
rollers wedge between the shaft and collar.
is a loss of air pressure on the parking diaphragm,
the output force on the shaft is reduced. However,
the shaft will not retract since its output force is
transferred to the mechanical lock mechanism.
RELEASE OF PARKING APPLICATION
brake actuator, it is necessary to re-apply air
pressure to equal a shaft force approximately the
same as was used in making the parking applica­
tion. This is necessary to release the locking roll­
ers so they can be moved away from the shaft
when air is re-applied to the locking piston. This
can be accomplished by making a 100 psi service
application after the push-pull control valve is "in"
to release the parking application.
the locking piston and the air on the parking dia­
phragm is exhausted. A full 100 psi service brake
application will be necessary to force the shaft
forward sufficiently to allow the locking rollers to
disengage and unlock the shaft. Upon release of the
service application, the return spring will return
the shaft to the release position.
system and it becomes necessary to move the
vehicle before service air can be restored, the
brakes may be manually released as follows:
parking reservoir by opening drain cock, then back
off the slack adjuster at each rear brake chamber.
If necessary, disconnect slack adjuster from cham­
ber push rod clevis.
To park, air is exhausted from the locking
NOTE: While in a parked position, when there
To release a parking application of the DD3
To release a parking application, air enters
In the event of a lo ss of air from the service
I MP ORT ANT : BLOCK WHEELS OF
VEHICLE BEFORE RELEASING BRAKES.
Exhaust any air pressure remaining in the