Chevrolet Light Duty Truck 1973, Service Manual

Page: 282 / 979

5 -1 6 BRAKES
is worn to within 1/32" of the shoe or rivet whichever is
applicable.
HYDRAULIC BRAKE FLUID
Brake fluid is a specially blended liquid which provides a
means of transmitting hydraulic pressure between the
master cylinder and the brake calipers and wheel
cylinders. Brake fluid is one of the most important parts
of the hydraulic system. Use GM Hydraulic Brake Fluid
Supreme No. 11 or DOT-3, meeting GM Specification
GM4653M or equivalent.
Brake fluid must have certain specific qualities such as:
1. Viscosity (free flowing at all temperatures).
2. High boiling point (remain liquid at highest
operating temperatures).
3. Non-corrosive (must not attack metal or rubber
parts).
4. Water tolerance (must be able to absorb and retain
moisture).
5. Lubricating ability (must lubricate piston and seals
to reduce wear and internal friction).
6
. Low freezing point (must not freeze even at lowest
operating temperatures).
If brake fluid becomes contaminated, it may lose some
of its original qualities. It is good practice to bleed the
brake system until all old fluid is removed when
performing major brake work. Old fluid should be bled
from the system and replaced with clean brake fluid if
any of the hydraulic system parts are corroded or the
fluid is discolored or dirty. If any of the rubber parts of
the hydraulic system are soft or swollen old fluid should
be removed and hydraulic system should be flushed with
clean brake fluid.
Do not
reuse old brake fluid at any
time.
In the event that improper fluid has entered the system,
it will be necessary to service the system as follows:
1. Drain the entire system.
2. Thoroughly flush the system clean with brake fluid.
CAU TIO N:
Use only brake fluid when flushing
a system.
3.
Replace all rubber parts of the system, brake hoses
and combination valve.
4. Refill the system.
5. Bleed the system.
Flushing Brake Hydraulic System
It is recommended that the entire hydraulic system be
thoroughly flushed with clean brake fluid whenever new
parts are installed in the hydraulic system.
Flushing is also recommended if there is any doubt as to
the grade of fluid in the system or if fluid has been used
which contains the slightest trace of mineral oil.
Flushing is performed at each bleeder valve in turn, and
in the same manner as the bleeding operation except
that bleeder valve is opened
1
-
1/2
turns and the fluid is
forced through the lines and bleeder valves until it
emerges clear in color. Approximately one quart of fluid
is required to flush the hydraulic system thoroughly.
When flushing is completed at all bleeder valves, make
certain the master cylinder reservoir is filled to proper
level.
BLEEDING HYDRAULIC SYSTEM
The hydraulic brake system must be bled whenever, any
line has been disconnected or air has in some way
entered the system. Bleeding of brake system may be
performed by one of two methods — either pressure or
manual. Bleeder valves are provided at the calipers,
wheel cylinders and are also located on some master
cylinders.
Sequence for Bleeding Wheel Cylinders
It is advisable to bleed one valve at a time to avoid
allowing fluid level in reservoir to become dangerously
low. The correct sequence of bleeding is to bleed the
valve, either front or rear system, nearest master cylinder
first. This sequence expels air from lines and calipers or
wheel cylinders nearest the master cylinder first and
eliminates the possibility that air in a line close to the
master cylinder may enter a line farther away after it
has been bled.
CAU TIO N:
Do not perform bleeding operation
while any brake drum is removed or with a
caliper removed from the disc.
Pressure Bleeding (Figs. 29 and 30)
NOTE: Pressure bleeding equipment must be
of the diaphragm type. That is, it must have
a rubber diaphragm between the air supply
and the brake fluid to prevent air, moisture,
oil and other contaminates from entering the
hydraulic system.
1. Clean all dirt from top of master cylinder and
remove cylinder cover and rubber diaphragm.
NOTE: Make sure brake fluid in bleeder
equipment is at operating level and that the
equipment is capable of exerting 20 to 30 lbs.
hydraulic pressure on the brake system.
2. Install Brake Bleeder Adapter J-23518 (J-23339 for
frame mounted boosters) on master cylinder.
Connect hose from bleeder equipment to bleeder
adapter and open release valve on bleeder
equipment.
NOTE: The combination valve, located near
the master cylinder, must be held in the open
position while bleeding. This can be accom­
plished by installing Tool J-23709 with the
open slot under the mounting bolt and
pushing in on the pin in the end of the valve
LIGHT DUTY TRUCK SERVICE MANUAL