GMC 4000 Series Service Manual page 243

Sec. 5B
Page 236
AIR BRAKES
CAMSHAFT
Rear brake camshafts have constant lift, S-
type cams forged integral with shaft. Camshaft is
carried in a bushing in brake spider at outer end,
and in a bushing in camshaft and brake chamber
bracket at inner end. Camshaft and brake chamber
bracket is bolted to brake spider in conjunction
with the brake dust shield. Lubrication fitting in
bracket provides a means of lubricating both bush­
ings; space between bushings serve as a lubricant
reservoir. Seals, in outer end of camshaft bore in
brake spider and in inner end of bracket, retain
lubricant and exclude dirt. Apply lubricant at in­
tervals indicated in LUBRICATION (SEC. 0).
Brake drum installations are illustrated in
"Rear S uspension" in"REAR
(SEC. 4C), and in "Front Suspension" under "FRONT
HUBS AND BEARINGS" (SEC. 3D). By referring to
the illustrations in the above sections, methods of
replacing brake drums are readily discernible.
Whenever brake drums are removed for serv­
icing brakes, inspect drums. If found to be scored,
rough, or out-of-round,drums should be machined.
Machining or grinding of brake drums in­
creases the inside diameter of the drum and chang­
es the lining to drum fit. When machining drums,
The Stopmaster type brake differs from the
conventional "S" cam type brake in several re­
spects. The air chamber push rod is connected to
the brake shoes through a series of wedges, roll­
ers, and plungers rather than through a slack ad­
juster and camshaft. Stopmaster brakes are used
at the rear wheels only. Stopmaster brakes employ
STOPMASTER STANDARD
G M C S E R V I C E M A N U A L
BRAKE DRUMS
HUBS AND BEARINGS"
STOPMASTER BRAKES
SINGLE AXLE "FAIL-SAFE"
Figure 2 7 — Stopmaster Brake A ssem blies
CAUTION: Do not overlubricate camshaft, as
excess lubricant may be forced by the seals into
the brake drums.
Slack adjuster, mounted on splined end of cam ­
shaft, is held in place by a lock ring. A spacer is
used on each side of slack adjuster. On the brake
assembly shown in figure 25, the two spacers (11)
are the same; on some models equipped with offset
slack adjusters, the inner spacer is thicker than
the outer spacer. Whenever camshaft has been re­
moved, coat bushing surfaces with S-17 Special
Lubricant or equivalent before installing. After
installation, apply lubricant as directed in LUBRI­
CATION (SEC. 0), and adjust brakes as directed
under "Brake Adjustments."
it is recommended that the following maximum
oversizes not be exceeded:
(a) Drums with standard diameter up to 14"
can be machined up to .060" oversize.
(b) Drums with standard diameter over 14"
can be machined up to .080" oversize.
When it is found that machining to these max­
imum lim its does not provide a suitable braking
surface, discard the worn drum and replace with
a new standard drum.
DO NOT EXCEED THESE LIMITS. THIS IS
SAFETY PRECAUTION.
two different type air chambers; (1) a standard
air chamber which is operated by air pressure and
(2) a "Fail-Safe" air chamber which can be oper­
ated either by air pressure or by spring pressure.
There are three basic variations of Stopmaster
brakes shown in figure 27; standard, single axle
"Fail-Safe," and tandem axle "Fail-Safe."
TANDEM AXLE "FAIL-SAFE"