GMC 4000 Series Service Manual page 253

Sec. 5B
Page 246
AIR BRAKES
Figure 4 2 — Com pressing "Fail-Safe" Power Spring
REMOVAL AND DISASSEMBLY
Refer to figure 40 (also refer to figure 41 for
components).
C A U T IO N : BEFORE ATTEMPTING BRAKE
CHAMBER REMOVAL,
CURELY, SINCE PARKING BRAKE WILL NOT BE
APPLIED.
1. With air pressure in parking brake cham­
ber (brake released), loosen release bolt spring
lock and swing to one side; then turn release bolt
clockwise as far as it will go (approx. 18 turns)
to com press the power spring. Refer to figure 42.
2. Exhaust air pressure from parking brake
chamber by pulling parking brake knob out.
3. Disconnect air lines from both chambers.
4. Mark pressure housing and non-pressure
housing so that parts can be reassembled in the
same relative position. Loosen clamping ring by
unscrewing bolt. Hang clamping ring on non­
pressure plate tube.
5. Remove entire cap and spring assembly,
pressure housing, and rubber diaphragm (see fig.
43). Diaphragm plate rod and boot will remain in
the non-pressure housing.
. Mount pressure housing in vise.
6
G M C S E R V I C E M A N U A L
BLOCK WHEELS SE­
Figure 4 3 — Removing "Fail-Safe" Assem bly
NOTE: Do not tighten vise jaws enough to d is­
tort pressure from cap assembly. See figures 44
and for illustration of this operation and dimen­
4 5
sions to make a "strap" wrench.
7. Remove washer and O-ring from pressure
housing.
. Remove piston assembly from cap by pull­
8
ing straight out. At tim es the spring, when fully
compressed, cocks slightly. It is then necessary
to loosen the release bolt a few turns so that the
piston can be withdrawn easily.
9. The piston, retainer plate, seal and expand­
er are held together by rivets. If any part of this
assembly is defective the complete assembly must
be replaced. Details are not serviced separately.
10. Cap and spring assembly is serviced as a
unit. If any of the parts become damaged or need
replacing, it is necessary to replace the complete
cap and spring assembly.
CAUTION: Do not attempt to disassem ble the
cap and spring assembly. In the event any part of
this assembly is defective, the complete assembly
must be replaced. The component parts are not
serviced separately.
CLEANING AND INSPECTION
(Refer to Figs. 40 and 41)
1. It is recommended that all brake chambers
be removed, disassembled, inspected and thorough­
ly cleaned at the time brakes are relined or at one