Wheel Cylinders - GMC 4000 Series Service Manual

BRAKE PIPE DISTRIBUTION A N D SWITCH ASSEMBLY
All models with split brake system are equip­
ped with a brake pipe distribution and switch a s­
sembly (refer to fig. 11). The hydraulic brake lines
are routed from the master cylinder, through the
switch assembly, to the front and rear brakes. The
switch is wired electrically to the brake alarm
indicator light on the instrument panel. In the event
of fluid loss in either the main or secondary brake
system , the indicator light will come on when the
brakes are applied. If this light comes on during
vehicle operation, it is an indication that there is
a failure somewhere in the brake system . The
truck should be removed from service as soon as
possible and the trouble corrected.
This light also comes on when the ignition key
is held in the "START" position. As soon as the
key is released to the "ON" position, the light goes
out. The purpose of this is to assure the driver
that the warning light is not burned out.
BRAKE PIPE DISTRIBUTION AND
SWITCH ASSEMBLY REPLACEMENT
(Refer to Figure 11)
REMOVAL
1. Disconnect battery cable.
2. Disconnect electrical lead from pressure
differential switch.
3. Place dry rags below the switch to absorb
any fluid spilled during removal of switch.
4. Disconnect four hydraulic lines from con­
nections at switch. If necessary, loosen line con­
nections at main cylinder. Cover open line ends
with clean, lint-free material to prevent foreign
matter from entering the system .
5. Remove mounting screw and remove switch
from vehicle.
There are four different type wheel cylinders
used on vehicles covered by this manual. Front
brakes are either Type "F," "FA," or Duo-Servo.
Rear brakes are either Type "FR-3," "FR-3A,"or
Twin-Action.
Since wheel cylinders can be disassembled
and repaired without removing them from the ve­
hicle the procedures are covered in this manual
along with "Replacement" procedures.
G M C S E R V I C E M A N U A L
INSTALLATION
dust and lint. If any doubt exists, wash switch in
suitable solvent, and dry with air.
ket with mounting screw.
hydraulic brake lines and connect lines to switch.
If necessary, tighten brake line connections at
main cylinder.
manual.
TESTING DISTRIBUTION
AND SWITCH ASSEMBLY
key in ignition switch to "START" position.
reservoirs have enough fluid. Add if necessary.
rear wheel.
brake warning light should come on due to p res­
sure difference between front and rear system s.
Approximately 200-250 psi differential is needed
to operate brake light switch.
pedal. Refill rear reservoir, if needed.
from entering hydraulic system during checks on
switch.
switch is 24 months, or 24,000 miles, any time
major brake work is done, or any time brake oper­
ation is abnormal.

WHEEL CYLINDERS

TYPES "F" AND "FA"
Removal
directed under "Brake Shoe Removal."
nector at upper cylinder. Remove inlet connector
attaching connecting tube fitting to upper cylinder,
HYDRAULIC BRAKES
1. Make sure new switch is clean and free of
2. Place switch in position and secure to brac­
3. Remove protective material from open
4. Connect switch electrical lead.
5. Connect battery cable.
6. Bleed the brake system s as outlined in this
1. Determine if bulb is functioning by turning
2. Check main cylinder to make sure both
3. Turn key in ignition switch to "ON" position.
4. Open wheel cylinder bleed screw at one
5. Depress brake pedal and hold down. The
6. Close bleed screw, then release brake
NOTE: Caution should be taken to prevent air
7. The recommended interval for checking
WHEEL CYLINDER REPLACEMENT
1. Jack up axle and remove brake shoes as
2. Disconnect axle brake tube from inlet con­
Sec. 5A
Page 175