1993 GMC P35/P3500 VAN Brake Service (ONLINE REPAIR MANUALS)

Height Sensing Proportioning Valve

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 freely.
• Remove lever retaining nut and disconnect lever from valve shaft, Fig. .
• Rotate valve shaft to permit installation of correct adjustment gauge, Fig. , ensuring that D shape...

Brake Lines & Linings

Remove one of the front wheels and inspect the brake disc, caliper and linings.

Do not allow any oil or grease on the linings. It is recommended that all linings be replaced in sets when worn or damaged.

If the caliper is cracked or fluid leakage through the casting is evident, it must be replaced as a unit.

Brake Roughness

The most common cause of brake chatter on disc brakes is a variation in thickness of the disc. If roughness or vibration is encountered during highway operation or if pedal pumping is experienced at low speeds, the disc may have excessive thickne...

Single Diaphragm Type

Disassemble

• Remove boot, silencer, vacuum check valve, grommet and front housing seal, Fig. .
• Scribe a line across front and rear housing, then mount brake booster to brake mounting fixture tool No. J 23456 or equivalent, Fig. .
• Turn fixture handle counterclockwise to unlock housings.
• Remove return spring, power piston assembly and power piston bearing from rear housing, Fig. .
• Remove reaction body retainer, piston rod, reaction retainer and filter.
• Remove power piston and pushrod assembly as follows:
• While holding the pushrod down against a hard surface, ...

Removal

• Siphon ^2/3 of brake fluid from master cylinder reservoir serving front disc brakes.
• Raise vehicle, support on jack stands and remove front wheels.
• Bottom 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 not allow cal...

Disassemble

• Place accumulator compressor tool No. J-26889 or equivalent, around end of accumulator.
• Install nut onto stud as shown in Fig. , then depress the accumulator with a C-clamp.
• Remove retainer by using a punch.
• Release C-clamp and remove accumulator compressor tool.
• Remove accumulator, O-ring, retainer, plug and spring, Fig. .
• Remove retainer, output pushrod, baffle, piston return spring and retainer.
• Using a suitable saw, cut off eyelet of pedal rod, then remove the boot.
• Remove nut, bracket and bolts, then separate cover from housing.
• Remove piston assembly and seals...