1991 Chevrolet Lumina APV Brake Service (ONLINE REPAIR MANUALS)

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...

Refer to Fig. , when replacing power brake unit.

• Disconnect battery ground cable.
• Remove master cylinder as follows:
• Remove air cleaner assembly, then the reservoir lead wire and pressure limit valve wire at electrical connectors.
• Remove fluid from reservoir using a syringe.
• Disconnect and plug three brake fluid lines from master cylinder assembly.
• Remove heat protector bolt from master cylinder bracket.
• Remove two master cylinder ...

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...

This braking system Fig. is designed to reduce the occurrence of wheel lock-up during severe braking conditions. To accomplish this function, the system regulates hydraulic brake line pressure by using an Electronic Hydraulic Control Unit (EHCU) module. This control module has two types of valves. Isolation valves which maintain pressure to each front wheel separately and rear wheels combined. Dump valves which dump pressure to each front wheel separately and rear wheels combined. These valves are controlled by a microcomputer which is part of the EHCU module. During a severe brak...

BRAKE DRUMS

Any time the brake drums are removed for brake service, the braking surface diameter should be checked with a suitable brake drum micrometer at several points to determine if they are within the safe oversize limit stamped on the brake drum outer surface. If the braking surface diameter exceeds specifications, the drum must be replaced. If the braking surface diameter is within specifications, drums should be cleaned and inspected for cracks, scores, deep grooves, taper, out of round and heat spotting. If drums are cracked or heat spotted, they must be replaced. Gr...

SLOW BRAKE PEDAL RETURN

• Excessive seal friction in booster.
• Faulty spool action.
• Restriction in return line from booster to pump reservoir.
• Damaged input rod end.

GRABBY BRAKES OR BOOSTER CHATTERS/PEDAL VIBRATES

• Faulty spool action caused by contamination in ...