Master Cylinders; Bench Bleeding - GMC G Service Manual

5A-28 HYDRAULIC BRAKES
DESCRIPTION
There are two designs of master cylinders available
depending on the brake option.
One is a full cast-iron design incorporating a conven­
tional front to rear brake split (figure 27). The primary piston
provides the fluid pressure to the front brakes, while the
secondary piston provides the fluid pressure to the rear
brakes. If the pressure is lost from either system, the
remaining system will function to stop the vehicle.
The second style master cylinder is designed for use
with a system using the low-drag calipers (figure 28). In
addition to the standard master cylinder functions, a quick
take-up feature is included. This provides a large volume of
fluid to the wheels at low pressure with the initial brake
application. This large volume of fluid is needed to over­
come the clearance created by the seal retracting the pis­
tons into the front calipers and the spring retraction of the
rear drum brake shoes.
Figure 28 — Composite Master Cylinder
MASTER CYLINDER
REPLACEMENT
H Remove or Disconnect (Figure 29)
• Apply the parking brake.
1. Brake pipes.
• Cover the ends of the pipes to prevent dirt from
entering the system.

MASTER CYLINDERS

2. Mounting nuts.
3. Master cylinder.
• If the vehicle is equipped with manual brakes,
refer to "Brake Pedal Replacement" in this sec­
tion for the removal of the pushrod from the
pedal.
B Install or Connect (Figure
• Prior to installation, bleed the master cylinder. Refer
to "Bench Bleeding" in this section.
1. Master cylinder.
• If the vehicle is equipped with manual brakes,
refer to "Brake Pedal Replacement" in this sec­
tion for the installation of the pushrod to the
pedal.
NOTICE: See " Notice" on page 5A-1 of this
section.
2. Mounting nuts.
F & l Tighten
• Nuts to "Specifications" at the end of this
section.
3. Brake pipes.
4. Bleed the brakes. Refer to "Bleeding the Brake
Hydraulic System" in this section.
• Release the parking brake.

BENCH BLEEDING

The purpose of bench bleeding is to remove the air from
the master cylinder so that when it is installed on the vehi­
cle, the brake system bleeding time will be reduced.
1. Plug the outlet ports and mount the master cylinder in
a vise with the front end tilted slightly down.
2. Fill the reservoir with clean brake fluid.
Using a tool with a smooth rounded end, stroke the
primary piston about 25mm (1-inch) several times. As
air is bled from the master cylinder, with the outlets
plugged, the resistance to the primary piston travel
will not allow the full 25mm (1-inch) stroke.
3. Reposition the master cylinder in the vise with the
front end of the master cylinder tilted slightly up.
Again stroke the primary piston about 25mm (1-inch)
several times.
4. Reposition the master cylinder in the vise to the level
position. Loosen the plugs one at a time and push the
piston into the bore to force the air from the cylinder.
To prevent air from being sucked back into the cylin­
der, tighten the plug(s) before allowing the piston to
return to its original position.
5. Fill the reservoir.
Normal bleeding procedures should be followed after
the master cylinder is installed. Refer to "Bleeding the
Brake Hydraulic System" in this section.
29)