1991 GMC K1500 Pickup Brake Service (ONLINE REPAIR MANUALS)

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...

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...

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...

• 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 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. ,...

• On vehicles w/manual brakes, disconnect master cylinder push rod from brake pedal.
• On all models, disconnect all wire connectors from mas...