1990 GMC P35/P3500 VAN Brake Service (ONLINE REPAIR MANUALS)

REMOVAL

• Siphon two-thirds of brake fluid from master cylinder reservoir serving front disc brakes.
• Raise vehicle, support on jack stands and remove front wheels.
• Bottom the caliper piston in bore. Insert a screwdriver between inboard shoe and piston, then pry piston back into bore. The piston can also be bottomed in the bore with a large C-clamp, Fig. .
• Remove support key retaining screw.
• Drive caliper support key and spring from steering knuckle with a suitable drift and hammer, Fig. .
• Lift caliper from anchor plate and off rotor, Fig. . Hang caliper from coil spring with wire. Do n...

Certain general service procedures apply, regardless of whether the brakes are vacuum or hydraulically assisted. Only top quality, clean brake fluid should be used. More seals and valves are used with power brake systems than with ordinary brakes, so an inferior brake fluid will do much more damage. For the same reason, be sure all dirt is kept out of the system. Additionally, on models with hydraulic assist brakes, care must be taken not to mix the fluids of the booster hydraulic system and brake hydraulic system as the fluids are not compatible.

If the power unit fai...

The procedures to remove & install the brake pads are outlined under "Caliper Service," "Remova... "

REMOVAL

• Siphon enough brake fluid out of the master cylinder to bring fluid level to^1-3 full to avoid fluid overflow when the caliper piston is pushed back into its bore.
• Raise and support vehicle, remove front wheels.
• Using a C-clamp, as illustrated in Fig. , push piston back into its bore.
• Remove two mounting bolts and lift caliper away from disc.

DISASSEMBLE

• Disconnect hose from the caliper being careful not to lose copper washers.
• Remove caliper as outlined above.
• Drain brake fluid from caliper.
• Pad caliper interior with clean shop towels and apply compressed air to remove piston, Fi...

DESCRIPTION

These power brake units, Figs. and , are a combination vacuum-hydraulic brake booster of the vacuum suspended type which use engine intake manifold vacuum and atmospheric pressure for its power.

These units consist of a vacuum power section and an hydraulic master cylinder section. The vacuum power section contains a power piston with rolling diaphragm mechanism and power piston return spring.

The control valve is made up of an air valve and floating vacuum control valve assembly. The reaction mechanism consists of an hydraulic piston, reaction plate and a series...

BLEEDING EHCU MODULE

The EHCU Module should be bled after replacement or if trapped air is in the unit. If bleeding is required the module must be bled thoroughly before the wheel cylinders and calipers.

There are two bleeders on top of the unit that appear to be normal bleeders, Fig. . These are modulator bleeders and must remain closed when the unit is not pressurized.

The internal bleeders are on either side of EHCU module. The valves are used to open the internal passages within the EHCU module. Both bleed valves must be rotated ^1/4-^1/2 a turn counterclockwise before beginning t...