Chevrolet 70 1969 Series, Shop Manual

Page: 200 / 913

BRAKES 5A-7
MASTER CYLINDERS
DESCRIPTION
Double barrel master cylinders are used on
all models covered by this manual. The double
barrel cylinder on conventional models is mounted
on the engine side of the fire wall with three bolts.
O n tilt cab models, the double barrel cylinder is
mounted under cab floor on a bracket with three
bolts.
Double barrel master cylinders have a
bleeder valve in the brake bore.
FILLER CAPS
Filler cap on double barrel cylinders is rec­
tangular 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 allow
for expansion and contraction of fluid level as the
result of changes in temperature as well as changes
in fluid level as the result of normal brake oper­
ation. 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. 6).
Brake fluid level in all master cylinders should
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.
MAINTENANCE (ALL TYPES)
Filler caps on all master cylinders are de­
signed to vent the fluid reservoir without permit­
ting loss of fluid. Bypass port between cylinder
bore and reservoir and vent hole in filler cap must
be kept open to assure proper operation. An ob­
structed bypass port will prevent return of fluid to
reservoir, preventing full release of brakes. By­
pass ports may be obstructed by one of the follow­
ing causes:
1. Clogged with dirt — remove master cyl­
inder and disassemble 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 — adjust
push rod.
MASTER CYLINDER REPLACEMENT
C O N V E N T IO N A L M O D E L S
Removal
1. Place a suitable container under master
cylinder to catch fluid when hydraulic lines are
disconnected. D O N O T RE-USE THIS F L U ID .
2. Disconnect hydraulic lines from outlets of
brake and clutch cylinders.
3. Pull push rod boots off master cylinder
tubes which extend through dash. Boots will re­
main on push rods when master cylinder is re­
moved.
4. Remove three bolts and lock washers at­
taching master cylinder to dash (nuts are welded
to inner side of dash) and remove master cylinder
assembly.
Installation
1. Place gasket over each cylinder tube. Posi­
tion master cylinder assembly at dash, while an
assistant inside cab guides push rods into pistons.
Attach cylinder to dash with three bolts and lock
washers. Tighten bolts firmly.
2. Connect hydraulic lines to brake and clutch
cylinder outlets.
3. Inside cab, place push rod boots over ends
of cylinder tubes.
4. Adjust push rods as previously directed
under "Brake Pedal and Linkage."
5. Fill master cylinder reservoir and bleed
brake system as previously directed under "Bleed­
ing Brakes."
T IL T CAB M O D E L S
Removal
1. Tilt cab forward and place a suitable con­
tainer under master cylinder to catch fluid when
hydraulic lines are disconnected.
D O N O T RE-USE THIS F L U ID .
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.
CHEVROLET SERIES 7 0 -8 0 H EA V Y DUTY TRUCK SH O P M A N U A L