1990 GMC S15 Jimmy 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...

REMOVAL

• Siphon two-thirds of brake fluid from master cylinder reservoir serving front disc brakes.
• Raise vehicle, support on jack stands and remove front wheels.
• Bottom the caliper piston in bore. Insert a screwdriver between inboard shoe and piston, then pry piston back into bore. The piston can also be bottomed in the bore with a large C-clamp, Fig. .
• Remove support key retaining screw.
• Drive caliper support key and spring from steering knuckle with a suitable drift and hammer, Fig. .
• Lift caliper from anchor plate and off rotor, Fig. . Hang caliper from coil spring with wire. Do n...

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...

WIRING DIAGRAMS

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

DIAGNOSTIC CIRCUIT CHECK

Refer to Figs. and for diagnostic circuit checks.

READING CODES

Trouble codes may be read with the TECH 1 scanner or by jumping terminal "H " of the ALDL to terminal "A" of the ALDL while observing the flashing of the "Anti-Lock" warning lamp. The terminals must be jumped for a few seconds before the code will begin.""

DIAGNOSTIC TESTS

CLEARING CODES

If the "Anti-Lock" warning lamp stays lit, the codes may be cleared with the TECH 1 scanner or by the following procedu...

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. ...

DELCO MORAINE TYPE

COMPOSITE TYPE

Refer to Figs. and when performing the following procedures.

Disassemble

• Remove master cylinder from vehicle.
• Remove reservoir cover and diaphragm. Discard old brake fluid in reservoir.
• Inspect cover and diaphragm. Replace if cut, cracked or deformed.
• Remove fluid level switch and proportioner valve assembly, if equipped.
• Depress primary piston and remove lock ring.
• Plug primary fluid outlet (outlet nearest to cowl when master cylinder is installed), then apply compressed air into secondary fluid outlet to remove primary and secondary pistons.
• R...