GMC 4000 Series Service Manual page 177

Sec. 5A
Page 170
HYDRAULIC BRAKES
3. Pull back boot on push rod. Visually check
to see if piston is seated firmly against piston snap
ring in cylinder bore. If piston is NOT against snap
ring, push rod is too long and should be shortened.
4. To adjust, loosen lock nut and turn rod in
or out of rod end as necessary. Adjust so that a
slight movement of brake pedal at pad is necessary
before end of push rod contacts piston.
5. Tighten lock nut.
6. Check operation of brakes.
P4500 (Refer to Fig. 8)
1. Loosen lock nut on push rod.
2. Turn push rod in or out of push rod end as
necessary so that free pedal travel measured at
center of pedal pad is between 0.060" and 0.250".
DESCRIPTION
There are four different type brake master
cylinders used on these vehicles; single barrel
(models with mechanical clutch controls), double
barrel (models with hydraulic clutch controls),
split system , and those which have a master cyl­
inder integral with the dash mounted type vacuum
power booster.
On conventional cab models, all types are
mounted on the engine side of the fire wall. The
single barrel cylinder is fastened by two bolts; the
double barrel cylinder uses three bolts. The boos­
ter type cylinder has four studs as part of the vac­
uum chamber. These studs pass through the fire
wall and the assem bly is fastened with four nuts
on these studs, inside the cab. The master cylinder
is fastened to the booster assembly on two studs
with nuts.
On tilt cab models the double-barrel cylinder
is mounted under the cab on a bracket with three
bolts.
On "P" Models the double-barrel cylinder is
mounted on a dash panel with three bolts.
On "S" Models the split system type master
cylinder is used. It is mounted on engine side of
the fire wall and is fastened by four bolts.
Some master cylinders have bleeder screws
and others do not (refer to brake bleeding sequence
chart, figure 6).
FILLER CAPS
Filler cap on single and double barrel cylin­
ders is rectangular and incorporates a combination
seal and diaphragm made of rubber. The purpose of
this is to prevent dirt from entering the reservoir,
to prevent brake fluid from leaking out and to al­
low for expansion and contraction of fluid level as
G M C S E R V I C E M A N U A L
ALL MODELS WITH DASH MOUNTED
POWER CYLINDER (STD. OR OPT.)
(Refer to Fig. 20)
necessary to obtain free play of 0.250" before push
rod causes
pedal pad. The push rod is part of the power cyl­
inder assembly and the free play is related to the
power cylinder not the brake pedal.
MASTER CYLINDERS
the result of changes in temperature as well as
changes in fluid level as the result of normal brake
operation. These caps are fastened to the top of the
master cylinder by a single bolt down through the
center which fits in a threaded hole in the master
cylinder body (see fig. 12).
integral with a dash mounted vacuum booster are
identical to caps used on regular master cylinders
except that they are round instead of rectangular.
covering both reservoirs and is fastened by a single
bolt in the center as shown in figure 13.
be checked periodically by removing filler cap and
making visual inspection. On conventional models
the master cylinder is accessible by lifting the
hood to the engine compartment on the left side of
vehicle. On tilt cab models the master cylinder is
accessible by removing the metal plate on the
riser panel below the front of the driver's seat.
On "P" Models the master cylinder is accessible
by removing a metal cover from outside the ve­
hicle at the front, or from inside the vehicle, on
the dash, between windshield and instrument panel.
Location differs with body manufacturer. On "S"
Models the master cylinder is accessible by lifting
the hood to the engine compartment on the left side
of the vehicle.
signed to vent the fluid reservoir without perm it­
ting lo ss of fluid. By-pass port between cylinder
bore and reservoir and vent hole in filler cap must
be kept open to assure proper operation. An ob­
structed by-pass port will prevent return of fluid
to reservoir, preventing full release of brakes.
3. Tighten lock nut.
4. Check operation of brakes.
1. Loosen lock nut on push rod.
2. Turn push rod in or out of push rod end as
movement of power cylinder hub.
NOTE: The free play is "at push rod" NOT at
3. Tighten lock nut.
4. Check operation of brakes.
F iller caps used on those cylinders which are
Filler cap used on split system is a single cap
Brake fluid level in all master cylinders should
M AINTENAN CE (ALL TYPES)
Filler caps on all master cylinders are de­