GMC 4000 Series Service Manual page 178

B y-pass ports may be obstructed by one of the fol­
lowing causes:
1. Clogged with dirt - - remove master cyl­
inder and disassem ble and clean all parts.
2. Swollen primary cup due to the use of
wrong fluid - - overhaul master cylinder, drain
and flush entire brake system , and refill with
proper fluid.
3. Pedal binding on shaft, preventing full re­
turn of piston - - free up and lubricate pedal.
4. Improper push rod adjustment
push rod.
MASTER CYLINDER REPLACEMENT
TILT CAB MODELS
Removal
1. Tilt cab forward and place a suitable con­
tainer under master cylinder to catch fluid when
hydraulic lines are disconnected. DO NOT RE­
USE THIS FLUID.
2. Disconnect hydraulic lines from outlets of
brake and clutch cylinders.
3. Pull push rod boots off master cylinder.
Boots will remain on push rods when master cyl­
inder is removed.
4. Remove three bolts attaching master cyl­
inder to support bracket and remove master cyl­
inder assembly.
Installation
1. Position master cylinder assembly at sup­
port bracket and guide push rods into pistons. At­
tach cylinder with three bolts. Tighten bolts firmly.
2. Connect hydraulic lines to brake and clutch
cylinder outlets.
3. Place push rod boots over ends of master
cylinder brackets.
4. Adjust push rods as directed under "Brake
Pedal and Linkage."
5. Fill master cylinder reservoir and bleed
brake system as directed under "Bleeding Brakes."
CONVENTIONAL MODELS
(SEPARATE MASTER CYLINDER)
Removal
1. Place a suitable container under master
cylinder to catch fluid when hydraulic lines are
disconnected. DO NOT RE-USE THIS FLUID.
2. Disconnect hydraulic lines from outlet of
brake cylinder (also clutch lines on dual cylinders).
3. Pull push rod boot off master cylinder tube
which extends through dash. Boot will remain on
push rod when master cylinder is removed.
4. Remove three bolts and lock washers (two
on single barrel cylinders) attaching master cyl-
G M C S E R V I C E M A N U A L
inder to dash (nuts are welded to inner side of dash)
and remove master cylinder assembly.
Installation
tion master cylinder assembly at dash, while an
assistant inside cab guides push rod into piston.
Attach cylinder to dash with three bolts and lock
washers (two on single barrel cylinders).
on dual) cylinder outlet.
- - adjust
cylinder tube.
Pedal and Linkage."
brake system as directed under "Bleeding Brakes."
CONVENTIONAL MODELS
(WITH DASH MOUNTED BOOSTER)
Removal
cylinder to catch fluid when hydraulic line is d is­
connected. DO NOT RE-USE THIS FLUID.
chamber and pull master cylinder off studs.
moved, omit Step 2. Instead, remove nut and bolt
which fastens push rod to brake pedal inside cab.
support bracket assembly to cab fire wall.
der and support bracket attached) away from fire
wall and remove from engine compartment.
Installation
inder and support bracket attached) in position on
fire wall from engine compartment side.
HYDRAULIC BRAKES
1. Place gasket over each cylinder tube. P o si­
2. Connect hydraulic line to brake (and clutch
3. Inside cab, place push rod boot over end of
4. Adjust push rod as directed under "Brake
5. Fill master cylinder reservoir and bleed
1. Place a suitable container under master
2. Remove two nuts from studs on booster
3. If complete booster assembly is to be re­
4. Remove four bolts which fasten booster and
5. Pull booster assembly (with master cylin­
1. Place booster assem bly (with master cyl­
Sec. 5A
Page 171