Brake Pedal And Linkage - GMC 4000 Series Service Manual

SPLIT SYSTEM ("S" MODELS)
The split system on "S" Models consists of
two separate brake system s split as follows:
1. Main System - front wheel brakes plus one
cylinder on each rear wheel brake.
2. Secondary System - one cylinder on each
rear wheel brake.
The system on both models consists of a dash
mounted master cylinder and two frame mounted
power cylinders.
It does not matter which system (main or se c ­
ondary) is bled first, however, each must be bled
separately. Because the two system s have a com­
mon push rod and act simultaneously, THE SPLIT
BRAKE SYSTEM CANNOT BE BLED MANUALLY.
THE PRESSURE BLEEDING METHOD MUST BE
USED.
Figure 6 illustrates a tool which can be made
locally for use in p ressure bleeding the split brake
system (both standard and optional).
Tool (J-23339) may also be used for this pur­
pose. It is a special adapter designed to fit the
m aster cylinder used on the "S" Models. This tool
is not illustrated.
PRESSURE BLEEDING
Refer to figure 7 for bleeding sequence.
1. Make sure fluid level in pressure tank is
up to petcock above outlet and that tank is charged
with 40 to 50 psi air pressure.
2. Clean dirt from around master cylinder
filler cap or cover. Remove standard cover and
install special cover required to fit the model cyl­
inder used. Connect pressure tank hose to filler
There are three basic types of brakepedaland
linkage installations used on vehicles covered by
this manual. They are illustrated in figures 8, 9,
and 10.
Figure 8 illustrates the type installation used
with a dash mounted master cylinder and a remote
booster cylinder (P4500 and 5500 Models).
Figure 9 illustrates the type installation used
with a dash mounted combination master and boos­
ter cylinder (4500 Series except "P," 5500 Series,
and 6500 Series).
Figure 10 illustrates the type installation used
with the master cylinder mounted under the cab
and a push through brake pedal (all tilt cab models).
BRAKE PEDAL REPLACEMENT
(ALL EXCEPT TILT CAB)
These models have a suspended type brake
G M C S E R V I C E M A N U A L
cap or cover opening. Bleed air from hose before
tightening connection. Open valves at both ends of
hose.
where used. Slip end of bleeder hose over bleeder
valve No. 1 and place other end in a glass jar con­
taining enough hydraulic fluid to cover end of hose.
Open bleeder valve with wrench and observe flow
of fluid from hose. Close bleeder valve as soon as
bubbles stop and fluid flows in a solid stream.
used), then bleed wheel cylinders in sequence shown
in figure 7. After bleeding wheel cylinders, repeat
bleeding operations at power cylinder (whereused).
s a tisfa c to r y , it is recom m en d ed that the re sid u a l
check valve in the master cylinder or the check
valve in the power cylinder piston be inspected (on
those models which have check valves). Improper
operation of either or both of these valves will re­
sult in the same pedal "feel" as air in the system .
Refer to applicable procedures for repair. If these
valves are operating properly, or if "feel" is not
satisfactory on models not using check valves, then
air is still present in system and bleeding again
will be necessary.
MANUAL BLEEDING
bleeding, except that the brake fluid is forced
through the lines by pumping the brake pedal in­
stead of by air pressure. Fluid in master cylinder
must be replenished after bleeding at each valve.
Brake pedal should be pumped up and down slowly,
and should be on downstroke as valve is closed.
BRAKE PEDAL A N D LINKAGE
HYDRAULIC BRAKES
3. First bleed master cylinder valve on models
4. Bleed valve No. 2 (on power cylinder where
5. If, after bleeding, the pedal "feel" is not
Manual bleeding is the same as pressure
Figure 8 — Brake Pedal and Master Cylinder
Installation (P4500 and 5500 M odels)
Sec. 5A
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