1990 GMC G35/G3500 VAN Brake Service (ONLINE REPAIR MANUALS)

EXCEPT TRACKER & 1990-91 C & K SERIES w/NON-SERVO BRAKES

• Raise and support vehicle, then remove tire and wheel assembly.
• Remove brake drum. If brake lining is dragging on brake drum, back off brake adjustment by rotating adjustment screw. Refer to individual truck chapter for procedure. If brake drum is rusted or corroded to axle flange and cannot be removed, lightly tap flange to drum mounting surface with a suitable hammer.
• Using brake spring pliers or equivalent, unhook primary and secondary return springs, Figs. through . Observe location of brake parts being removed to aid during ins...

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...

The height sensing proportioning valve must be adjusted whenever the valve and/or linkage is disassembled or removed for service. In addition, adjustment should be checked if excessive front wheel lock-up is experienced with a lower than desired brake application rate when vehicle is at or near maximum GVWR.

• Raise and support vehicle leaving wheels on, and allow axle to hang free (no load condition).
• Remove lever retaining nut and disconnect lever from valve shaft, Fig. .
• Rotate valve shaft to permit installation of correct adjustment gauge, Fig. ,...

Diagnose power brakes systems as if two separate systems exist. Check for faults in the regular brake system first. If no problem exists in the regular brake system, diagnosis the power brake circuit is necessary. For a quick check of proper power unit operation, press the brake pedal firmly and then start the engine. The pedal should fall away slightly and less pressure should be needed to maintain the pedal in any position. On vacuum suspended power units, air will rush into the air intake when the brakes are applied.

To perform another check, remove the stop light switch, ...

In order to properly service power brake systems, a thorough understanding of the various power assist systems is necessary. These systems provide a mechanical assist, acting to increase the force applied to the master cylinder piston by the driver while decreasing the effort necessary to obtain acceptable stopping performance. Two basic types of power assist mechanisms are used: vacuum assist diaphragm assemblies which use engine vacuum and in some cases vacuum from an external vacuum pump, and hydraulic assist assemblies which use hydraulic pressure developed by an external pump (...

CABALLERO & EL CAMINO

Pump brake pedal several times with engine off to deplete accumulator of fluid.

• Remove two nuts attaching master cylinder to booster, then move master cylinder away from booster with brake lines attached.
• Remove three hydraulic lines from booster. Plug and cap all lines and outlets.
• Remove retainer and washer securing booster pushrod to brake pedal arm.
• Remove four nuts attaching booster unit to dash panel.
• From engine compartment, loosen booster from dash panel and move booster pushrod inboard until it disconnects from brake pedal arm. Remove spring wa...