1990 GMC G15/G1500 VAN Brake Service (ONLINE REPAIR MANUALS)

DESCRIPTION

These units, Figs. and , have a vacuum power chamber that consists of a front and rear shell, a housing divider, front and rear diaphragm and plate assemblies, an hydraulic pushrod and a diaphragm return spring.

The unit operates in much the same manner as the Single Diaphragm unit described above. The diaphragm and plate assemblies use the pressure differential created by engine intake manifold vacuum and atmospheric pressure to assist the hydraulic pushrod.

BOOSTER SERVICE

Disassemble

• Remove boot, silencer, vacuum check valve, grommet and front housing seal Fig. ...

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...

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...

• Raise and support vehicle.
• Remove wheel, drum and brake shoes.
• Disconnect hydraulic line at wheel cylinder. Do not pull metal line away from cylinder, as this may kink or bend line. Line will separate from cylinder when cylinder is moved away from brake backing plate.
• Remove wheel cylinder-to-brake plate attaching screws, then the wheel cylinder.
• Remove boots, pistons, springs and cups from cylinder, Fig. .
• Clean all parts with brake fluid.
• Inspect cylinder bore. A scored bore may be honed as long as the diameter is not increased by ...

The Rear Wheel Anti-Lock (RWAL) system, Fig. , reduces the occurrence of rear wheel lockup during severe braking by regulating rear hydraulic line pressure. Pressure applied to the rear wheel brakes is regulated by a control valve assembly located under the master cylinder. The control valve consists of a dump valve which releases hydraulic pressure into an accumulator, and an isolation valve which holds rear brake pressure. Operation of the dump valve and isolation valve is controlled by an Electronic Control Unit (ECU) which is mounted next to the master cylinder.

The ECU receives ...

REMOVAL

• Remove caliper assembly as outlined above.
• Remove inboard pad with retainer spring. Dislodge outboard pad and position caliper on the front suspension so the brake hose will not support the weight of the caliper.
• Remove two sleeves from inboard ears of the caliper.
• Remove four rubber bushings from the grooves in each of the caliper ears.

INSTALLATION

• Lubricate new sleeves, rubber bushings, bushing grooves and mounting bolt ends with Delco Silicone Lube or equivalent.
• Install new bushings and sleeves in caliper ears. Position the sleeve so that the end toward the pad is flush...