1990 Chevrolet V1500 Suburban Brake Service (ONLINE REPAIR MANUALS)

HARD PEDAL

• Internal vacuum leak.
• Faulty control valve.
• Broken or damaged hydraulic brake lines.
• Collapsed or damaged vacuum hose.
• Plugged or loose vacuum fitting.
• Bad stud welds on front or rear housing on power head.
• Faulty booster diaphragm.
• Restricted air filter element.
• Worn or distorted reaction plate or levers.
• Cracked or broken pow...

EHCU MODULE

The EHCU module is not serviceable and must be replaced as a unit. Refer to Fig. when replacing the unit.

• Bleed brake booster by depressing brake pedal several times.
• Remove washer tank attaching bolts, then position aside.
• Remove EHCU electrical connectors.
• Disconnect module brake lines. Use caution not to get brake fluid on paint or electrical connections.
• Remove upper to lower bracket assembly attaching bolts.
• Remove EHCU module and upper bracket assembly from vehicle.
• Remove upper mounting bracket to EHCU module attaching bolts.
• Reverse procedure to install, noting...

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 BRAKES

Manual Bleeding

• If vehicle is equipped with power brakes, remove vacuum reserve by applying brakes several times with engine off.
• Fill master cylinder reservoirs with suitable brake fluid. Be sure to always keep master cylinder reservoirs at least half full during entire bleeding procedure.
• If master cylinder is suspected to have air in the bore, it must be bled first, as follows:
• Disconnect forward brake pipe connection at master cylinder.
• Allow brake fluid to fill master cylinder bore until it begins to flow from forward pipe connector port.
• Connect and tighten forward...

DISASSEMBLE

• Remove pushrod clevis and locknut.
• Install brake booster to brake mounting fixture, Fig. . Ensure booster vacuum pipe is clear of mounting fixture base and the booster mounting nuts are not over tightened.
• Turn fixture handle until No. 1 and No. 2 body tabs are aligned, Fig. .
• Remove booster from fixture, then separate body No. 1 and No. 2. This step must be performed with care because there is spring pressure between body No. 1 and No. 2.
• Remove booster piston spring, piston rod, boot, air cleaner and air cleaner separator from body No. 2.
• Remove valve stopper key cush...

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, ...