On-Vehicle Service; Bleeding The Brake Hydraulic System - GMC G Service Manual

BLEEDING THE BRAKE
HYDRAULIC SYSTEM
A bleeding operation is necessary if air has been intro­
duced into the hydraulic brake system.
It may be necessary to bleed the system at all four
wheels if air has been introduced by a low fluid level condi­
tion in the master cylinder or if the brake pipes have been
disconnected at either the master cylinder or the combina­
tion valve. If a pipe is disconnected at one wheel, then only
bleed that particular wheel.
The time required to bleed the hydraulic system when
the master cylinder is removed can be reduced if the mas­
ter cylinder is filled with fluid and as much air as possible is
bled from the cylinder before installing it on the vehicle.
MANUAL BLEEDING
If the vehicle is equipped with power brakes, deplete the
vacuum reserve by applying the brakes several times with
the engine off.
Use extreme care to prevent brake fluid from contacting
any painted surface.
1. Fill the master cylinder reservoirs with brake fluid
specified in MAINTENANCE AND LUBRICATION
(SEC. OB).
• Maintain the. fluid level in the reservoir during
the bleeding operation.
2. If the master cylinder is suspected to have air in the
bore, it must be bled before any wheel cylinder or
caliper.
• Remove the forward brake pipe connection at
the master cylinder.
• Allow brake fluid to flow from the connector port.
• Connect the brake pipe but do not tighten.
3. Slowly depress the brake pedal, allowing the air to
bleed from the loose fitting.
• Tighten the fitting before releasing the pedal.
• Wait 15 seconds and repeat this sequence,
including the 15-second wait, until all the air is
purged from the bore.
4. After all the air has been removed from the forward
connection, disconnect the rear pipe.
• Allow brake fluid to flow from the connector port.
• Connect the brake pipe but do not tighten.
5. Slowing depress the brake pedal, allowing the air to
bleed from the loose fitting.
• Tighten the fitting before releasing the pedal.
• Wait 15 seconds and repeat this sequence,
including the 15- second wait, until all the air is
purged from the bore.
ON VEHICLE SERVICE
HYDRAULIC BRAKES 5A-5
6. If it is known that the calipers and wheel cylinders do
not contain any air, then it will not be necessary to
continue; otherwise, bleed each wheel in the follow­
ing sequence:
• Right rear
• Left rear
• Right front
• Left front
7. Attach a hose to the wheel cylinder/caliper bleeder
screw.
• Immerse the opposite end of the hose into a
container partially filled with clean brake fluid.
8. Slowly depress the brake pedal one time and hold.
• Loosen the bleeder screw to purge the air from
the wheel cylinder/caliper.
• Tighten the bleeder screw and slowly release
the pedal.
• Wait 15 seconds, then repeat this sequence,
including the 15- second wait, until all the air is
purged from the wheel cylinder/caliper.
9. Continue steps 7 and 8 at each wheel until the entire
brake system has been bled.
10. Check the brake pedal for "sponginess" and the brake
warning lamp for an indication of unbalanced pres­
sure. Repeat the entire bleeding procedure to correct
either of these two conditions.
PRESSURE BLEEDING
The pressure-bleeding equipment must be of the dia­
phragm type. 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.
Also, adapters are needed depending on the type of master
cylinder used.
Tools Required:
J 29567 Brake Bleeder Adapter (Plastic
Reservoir)
J 23518-01 Brake Bleeder Adapter (Cast-Iron
Reservoir)
J 23709 Combination Valve Depressor
NOTICE: It is very important that the correct
master cylinder bleeder adapter be used to
avoid possible damage to the master cylinder
reservoir.
1. Fill the pressure tank at least 1/3 full of brake fluid.
The bleeder must be re-bled each time fluid is added.
• Charge the bleeder to 140-170 kPa (20 to 25
psi).