GMC 1976 ZEO 6083 Maintenance Manual page 234

time to avoid allowing fluid level in reservoir to
become dangerously low. For the proper sequence
refer to Figure 1 .
Do not perform bleeding operation while any
brake drum or disc pad is removed.
BLEEDING WHEEL CYLINDER WITHOUT
PRESSURE TANK
l . Fill master cylinder.
2 .
Install bleeder wrench on bleeder valve. Slip a
brake bleeder hose over ball of wheel cylinder bleeder
valve (See figure 2) . Place lower end of bleeder tube
in a glass jar that is partially filled with clean brake
fluid. Position end of tube so that it will remain
submerged under fluid during bleeding operation.
Unscrew bleeder valve
3/4
of a turn .
Attach
J-23709
to the combination valve (figure
3)
to hold the push rod in and allow the pressure in
the line to flow through the valve requiring less pres-
sure to bleed the front cylinders.
3 . Depress brake pedal a full stroke, close bleeder
valve, then allow pedal to return slowly to released
position . Allowing pedal to return quickly may draw
air into system . Continue operating pedal in this
manner until fluid flows from bleeder tube into glass
jar in a solid stream that is free of air bubbles, then
close the bleeder valve securely and remove bleeder
tube and wrench .
4 . Frequently check master cylinder to make sure
that it contains fluid. Allowing reservoir to be empt-
ied will cause air to be drawn into hydraulic system .
5 . When bleeding operation is completed at all
wheel cylinders where needed, make sure that fluid
level is no more than
1/4"
below lip of reservoir, then
install rubber diaphragm and cover.
6. Discard the brake fluid deposited in glass jar
during bleeding operation.
BLEEDING WHEEL CYLINDER WITH
PRESSURE TANK
When using a pressure tank, air bubbles may
form in the tank and enter the brake hydraulic sys-
tem. To avoid this, observe the following points
when handling a pressure tank : (1) Do not shake or
agitate the pressure tank after air pressure has been
added or is being added. (2) Allow pressure tank to
stand in one position as much as possible and bring
air hose over to tank when adding head of air. (3)
Make certain the valves on the pressure tank lines are
not defective, allowing air to be sucked in when fluid
BRAKES
5- 1 5
passes through the lines. (4) Pressure tank should be
kept at least
1/3 full of fluid to avoid air bubbles
forming. (5) If pressure tank is full of air bubbles,
release air pressure and those bubbles will increase in
size, be forced to top of fluid and escape .
It is recommended that pressure bleeding equip-
ment 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
contaminants from entering the hydraulic system .
1 . Thoroughly clean master cylinder reservoir
cover and surrounding area ; then remove cover and
diaphragm.
2 . Make sure that pressure tank is at least
1/3
full
of specified brake fluid and that hose and master
cylinder reservoir are filled with fluid . Attach hose
to master cylinder reservoir adapter.
3 .
Install Bleeder Wrench on bleeder valve. Slip
a brake bleeder tube over ball of wheel cylinder
bleeder valve (figure 2) . Place lower end of bleeder
tube in a clean glass jar. Unscrew bleeder valve
3/4
of a turn .
Attach
J-23709
to the combination valve to hold
the push rod in and allow the pressure in the line to
flow through the valve (figure 3) .
4 .
Open pressure tank hose valve to apply fluid
to master cylinder under pressure that does not ex-
ceed
35
pounds . It is not necessary to pump the brake
pedal when using pressure tank .
5 .
When fluid flows from bleeder tube into glass
jar in a solid stream that is free of air bubbles, that
particular cylinder and line are bled ; tighten bleeder
valve securely and remove bleeder tube .
6. When bleeding operation is completed at all
wheel cylinders, where needed, make sure that fluid
level is
1/4"
from the lowest portion of the top of
each reservoir. Install rubber diaphragm and cover.
FLUSHING BRAKE HYDRAULIC SYSTEM
It is recommended that the entire hydraulic sys-
tem 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 wheel cylinder in