GMC 4000 Series Service Manual page 236

PREVENTIVE MAINTENANCE
DD3 BRAKE ACTUATOR
Depending on experience and type of operation,
the drain slot in the actuator non-pressure plate
should be checked and cleared periodically.
Brakes should be adjusted as is customary
with any brake chambers. Push rod travel should
be as short as possible without brakes dragging.
Excessive travel not only shortens the normal
service life of diaphragms but gives slow braking
response, wastes air, and decreases brake torque
output.
Push rod to slack adjuster alignment should be
checked in both the applied and released positions,
the rod should move out and return properly with­
out binding. Also, check the angle formed by the
slack adjuster arm and push rod. It should be 90
or greater in any position,
It is recommended that every year, or after
50,000 m iles whichever occurs first (and also
dependent upon type of operation, and operator
experience) that brake actuators be removed from
vehicle and disassembled.
Disassem ble DD3 brake actuator, clean all
parts and lubricate locking mechanism. A special
barium base grease (or its equivalent) is recom ­
mended as a lubricant.
When diaphragms or return spring or both
are replaced, like parts in the corresponding
brake actuator on the same axle should also be
replaced.
OPERATING A N D LEAKAGE CHECKS
DD3 BRAKE ACTUATOR
OPERATING
With the brake actuator in the released posi­
tion, make several service brake applications and
note that actuators apply and release properly.
Operate parking push-pull control valve and ob­
serve that actuators apply. While actuators are
in a parking position, drain air supply to parking
diaphragm and note that actuators remain applied.
Replenish air supply to auxiliary diaphragm.
Operate push-pull control valve to release parking
application, then make a 100 psi service applica­
tion to complete release of actuators. The magni­
tude of the service brake application to release the
brakes may vary on different vehicles due to com­
pressor governor setting. A service application of
approximately 100 psi will release the brakes.
LEAKAGE
With air system at maximum governor p res­
sure and DD3 brake actuators in the released
position, check drain slot and around the push rod
boot with a soapy solution to detect possible leak­
age past the locking piston grommet.
G M C S E R V I C E M A N U A L
again check the actuator drain slot for service
diaphragm leakage. Continue to hold the service
application and coat around the service and park­
ing diaphragm clamping rings with the soapy solu­
tion to detect seal leakage.
ing position and check the exhaust port of a service
brake application to detect parking diaphragm
leakage. This parking diaphragm leakage detection
point could be the exhaust port of the foot brake
valve, quick release valve or relay valve. While
still in a parking position, the parking diaphragm
clamping ring should be coated with the soap solu­
tion to detect seal leakage.
rings in either of the above tests, the clamping
ring nuts should be tightened evenly but only enough
after adjustment.
to stop leakage.
described, or leakage is excessive, it is recom ­
mended that it be repaired or replaced.
REMOVAL
than air brakes.
position,
brake hoses from parking and service ports of
the actuator.
reservoir. This may be done by opening the drain
cock in the reservoir tank or by pulling out the
push-pull control valve button.
Make and hold a service brake application and
Operate the actuator control valve to a park­
Should leakage be detected at the clamping
If the DD3 brake actuator does not function as
DD3 BRAKE ACTUATOR
REPLACEMENT
(Refer to Figure 20)
1. Block and hold vehicle by some means other
2. With the brake actuators in the released
disconnect or completely remove air
3. At this point, exhaust air from parking
Figure 20— D D 3 Brake Actuator System
Installed at Rear Axle
Sec. 5B
Page 229
AIR BRAKES