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

WIRING DIAGRAMS

Refer to "Wiring Diagrams" in "Vehicle Systems."

READING TROUBLE CODES

Trouble codes are accessed by connecting a jumper wire between terminals A and H of the underdash ALDL connector and observing the flashes of the brake warning lamp. Note the following when performing the Diagnostic Circuit Check to access trouble codes.

• When the terminals are jumped, count the number of short flashes starting from the initial long flash and including the long flash in the count.
• The count sequence will repeat as long as the ALDL connector terminals are jumped with the ignition on.
• ...

BRAKE LINES & LININGS

Remove one of the front wheels and inspect the brake disc, caliper and linings.

Do not get any oil or grease on the linings. It is recommended that all linings be replaced in sets when worn or damaged.

If the caliper is cracked or fluid leakage through the casting is evident, it must be replaced as a unit.

BRAKE ROUGHNESS

The most common cause of brake chatter on disc brakes is a variation in thickness of the disc. If roughness or vibration is encountered during highway operation or if pedal pumping is experienced at low speeds, the disc may have excessive t...

MASTER CYLINDER

Some vehicles use a conventional tandem master cylinder with two hydraulic pistons operating in line. The primary piston controls the front brake system and the secondary piston controls the rear brake system.

Other models use a master cylinder that operates the same as the conventional master cylinder, but also incorporates a quick take-up feature in the rear chamber to reduce excessive pedal travel which may result from increased fluid displacement required to move the low drag caliper piston out against the rotor.

The quick take-up uses a spring-loaded ball...

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...

• Inspect components for damage or unusual wear. Replace as necessary.
• Inspect backing plate attaching bolts, and ensure they are tight.
• Inspect wheel cylinders. Excessive fluid indicates cup leakage and need for wheel cylinder replacement. A slight amount of fluid is always present and is considered normal, act...

DESCRIPTION

The Hydro-Boost system, Fig. , provides an additional cylinder in the brake system. This cylinder contains no brake fluid. The Hydro-Boost cylinder is hydraulically operated, by pressurized fluid from the power steering pump, providing power assist to operate a dual master cylinder brake system.

The booster is composed of two sections; the linkage section and the power section.

The booster is designed so that if a total absence of power assist occurs, the brakes can be applied manually, but somewhat greater pressure on the brake pedal is required.

DISASSEMBLE

• Place accumulato...