GMC 4000 Series Service Manual page 252

1 Spanner Nut Retaining W asher
2 Spanner Nut
3 Wedge Rod Guide
4 Tag Nut
5 Caution Tag
Nut
6
7 Lock Washer
Clamping Ring
8
ment is transmitted to both ends of each brake
shoe through wedges, rollers, and plungers which
are installed in the actuating housing in the brake
spider. One plunger for each shoe incorporates an
adjusting wheel to provide a means of adjusting
the brakes to compensate for normal lining wear.
All Stopmaster brakes have automatic adjuster
mechanisms.
SERVICEABILITY TESTS
OPERATING TEST
1. Service Brakes
Make a brake application and check expansion
of brake shoes against drum. Shoes should move
freely and instantly when brake treadle is de­
pressed. Release treadle. Shoes should contract
and release the brakes without any lag. Visually
inspect entire lining area to see if lining is bearing
properly on both sides.
2. Parking Brakes
Apply parking brakes by pulling knob of con­
trol valve to release air from chamber. Brakes
should apply promptly and should hold on any grade
on which vehicle is expected to operate. Release
the brakes by pushing knob in. The brakes should
release instantly and wheels turn freely.
LEAKAGE TEST
1. Service Brakes
With brakes applied, check air chambers for
leakage at clamping ring by covering ring with
soap suds. Also apply suds to drain holes on bot­
G M C S E R V I C E M A N U A L
9 W asher
10 Bolt
11 N on-P ressu re Housing
12 Push Rod Boot
13 Diaphragm Plate Rod
14 R u b b er D ia p h r a g m
15 "Fail-Safe" P ressu re Housing
16 Housing to Cap Rubber Washer
Figure 41 - "Fail-Safe" Air Cham ber Components
tom of chamber. Any small air leaks should be
evident. No leakage is perm issible at either loca­
tion. If leakage occurs at clamping ring, tighten
clamping ring. If leakage still persists, diaphragm
may be deteriorated or not fitting properly between
pressure and non-pressure housings (see figs. 40
and 41).
an indication that diaphragm is faulty and should
be replaced.
2. Parking Brakes (Fig. 40)
sure, apply soap suds on cap around release bolt.
Any leakage that is evident indicates a faulty piston
seal. Seal should be replaced and suds applied
again to be sure leakage has been corrected.
sure housing joint. If leakage is evident, fault
could be due to distorted cap caused by careless
handling or deteriorated rubber washer between
cap and housing.
with soap suds. Leakage at this point indicates
leakage past O-ring seal in the pressure housing.
If leakage occurs, replace O-ring with new part.
from the vehicle for inspection and replacement of
parts without removing the non-pressure housing
and without disturbing the mechanical actuating
mechanism.
17 Housing to Plunger "O" Ring
18 Air Chamber Piston Plunger
A ssem bly
19 Air Chamber Cap A ssem bly
20 R elea se Bolt Spring Lock
21 C apscrew
22 Lock W asher
If air is escaping from drain holes only, it is
a. With parking brake released by air p res­
b. Soap suds should be applied at cap to p res­
c. Coat service brake relay valve exhaust port
SERVICING "FAIL-SAFE" BRAKE
CHAMBERS
Brake chamber components can be removed
Sec. 5B
Page 245
AIR BRAKES
1 9 2 0