Chevrolet 70 1969 Series, Shop Manual

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BRAKES 5 A - 11
IM P O R T A N T : Only genuine G M C parts should
be used. Non-genuine parts may deteriorate and
swell in a short time, resulting in faulty brake
operation.
2. Inspect cylinder bore for scores, scratches,
or corrosion. Light scratches or slightly corroded
spots may be polished out with crocus cloth. Never
use emery cloth or sandpaper. If scratches or
corrosion are too deep to be polished out, replace
cylinder.
3. Check fit of new pistons in cylinder bore,
using a feeler gauge as shown in figure 10. Clear­
ance should be within 0 .0 0 1 " to 0.005" on "FR- 3"
type. Replace cylinder if clearance exceeds the
maximum.
Assembly
Before assembling wheel cylinder, be sure
each part has been cleaned in denatured alcohol.
C A U T IO N : Do not use kerosene or gasoline
for cleaning wheel cylinder parts. Dip each internal
part in hydraulic brake fluid before assembling.
1. Install pistons, piston cups, and spring,
with cup lips toward inside of cylinder.
2. Assemble push rods and boots, then install
on cylinder. Seat boots evenly in cylinder grooves
Align push rod slots as shown in figure 8 .
3. Install bleeder valve in upper cylinder.
HYDRAULIC LINES
Hydraulic brake system units are intercon­
nected by flexible hose and special metal tubing.
Flexible hose is used between master cylinder (on
cab) and frame connection, between frame and
front wheel cylinders, and between frame and rear
axle brake line. Whenever hydraulic lines have
been disconnected for any reason, brake system
must be bled, after connecting lines, as directed
under "Bleeding Brakes."
F L E X IB L E HOSE
At front wheels, hose fitting is threaded into
wheel cylinder connector, with a copper gasket used
between shoulder on hose fitting and connector.
Fitting at other end of hose is inserted through
hole in frame and secured by atoothedlock washer
and nut or by a spring lock. Brake tube connector
or tee fitting threads into end of hose fitting.
VACUUM PC
All models covered by this manual which have
hydraulic brakes have vacuum power cylinders as
standard equipment. Single axle conventional model
have a single diaphragm power cylinder as stand­
ard equipment, with a tandem diaphragm power
cylinder available as optional equipment. All other
Figure 9 — Checking Fit of Piston in Wheel Cylinder
At rear axle, fitting at one end of hose is
threaded into axle tee, with a copper gasket used
to seal the connection. Other end of hose is insert­
ed through frame bracket and secured by a toothed
lock washer and nut or by a spring lock. Brake
tube connector or tee fitting threads into end of
hose fitting.
To remove hose, disconnect end at frame or
frame bracket, then unscrew hose fitting from
wheel cylinder or rear axle tee. When installing
hose, always use a new copper gasket at wheel
cylinder and rear axle tee. When frame end of hose
is secured by a nut, always hold hose fitting with
a wrench while tightening nut to prevent twisting
hose.
M E T A L TUBING
When necessary to replace metal brake tubing,
always use special metal tubing which is designed
to withstand high pressure and to resist corrosion.
Ordinary copper tubing is not satisfactory for use
as hydraulic brake lines. When replacing tubing,
always use the same size as that removed.
TU BE F L A R IN G
In order to insure a proper flare, a special
flaring tool must be used. When using tool, in­
structions furnished by the tool manufacturer
should be followed. Always inspect newly formed
flares for cracks or malformations which might
cause leaks.
I CYLINDERS
models have tandem diaphragm power cylinders
as standard.
The power cylinder is a combined vacuum -
hydraulic power unit, utilizing vacuum and atmos­
pheric pressure for its operation.
CHEVROLET SERIES 70 -80 H EAVY DUTY TRUCK SH O P M A N U A L