BRAKE BALANCE PROCEDURE REAR WHEEL SLIDE (VAC. P/B)

1982 & 1983 S/T 15 PICKUPS AND JIMMYS WITH VACUUM POWER BRAKES (RPO J50)

S/T 15 Pickup Trucks and Jimmys are designed to carry loads, and therefore, they are equipped with brakes that are capable of good stopping performance in a fully loaded condition. When these vehicles are braked to wheel slide in a lightly loaded condition, it is normal for the rear wheels to slide first. The design in consistent with industry practice and sound engineering principles that General Motors and other light truck manufacturers have applied successfully for many years. Modulation of the brakes by the driver to avoid or relieve wheel slide is desirable to maintain steering and shorten stopping distance during braking.

Some customers who operate theirr S/T 15 Pick-ups or Jimmys lightly loaded most of the time may prefer less rear braking. If so, the following procedure is an acceptable method of reducing rear braking on 1982/1983 S/T 15 Pickups and Jimmys with vacuum power brakes. It will result in a higher rate of deceleration at wheel slide and shorter stopping distance in a lightly-loaded condition. However, the customer should be informed that there will be a reduction in brake performance when fully loaded, in that wheel slide will occur at a lower deceleration rate and minimum stopping distances will be increased. General Motors believes this trade-off is acceptable because 1984 S/T 15 Pickups and Jimmys with similar brake configurations meet all of the stopping distance requirements of Federal Motor Vehicle Safety Standard 105 for Light Duty Trucks 1982/1983 models, in both their original design condition or as modified by this procedure, meet the same requirements.

PROCEDURE

1. Verify that the vehicle has original equipment or authorized General Motors replacement linings, front and rear. Step 3 should not be used to alleviate a complaint that originated when aftermarket linings were installed.

2. If the rear linings are new, rear braking can generally be reduced somewhat by burnishing. GMC Truck has established a proper burnishing procedure which was described in Truck Service Bulletin 77-I-28, dated August, 1977.

Following is the information from Bulletin 77-I-28.

BURNISH PROCEDURE

General

In carrying out this procedure, use of a decelerometer is recommended. If a decelerometer is not available, the deceleration rates can be approximated by stopping the vehicle in the time periods indicated in the procedure.

All steps performed during warm-up and soak cycle of the procedure are to be made utilizing the parking brake system. To allow for proper apply modulation, it is essential that the parking brake lever be held in the released position during all parking brake applications.

NOTE: Since numerous stops will be made with only the parking brake system, and thus the brake lights not activated, a location with minimal traffic conditions should be selected, emergency flashers utilized, and caution exercised.

Warm-Up

Using the parking brake only, make 12 stops from 40 mph at a deceleration of 6 ft./sec. at 0.2 mile intervals.

NOTE: A 6 ft./sec deceleration is equivalent to bringing the vehicle to a halt from 40 mph in 10 seconds.

Soak Cycle

Immediately following the warm up cycle, using the parking brake only, make ten stops from 30 mph at a deceleration of 6 ft.sec at 0.5 mile intervals.

Note: A 6 ft/sec deceleration is equivalent to bringing the vehicle to a halt from 30 MPH in 7 seconds.

Check Procedure

After completion of the burnish procedure, test results by decelerating from 45 mph at 20 ft./sec utilizing service brakes; rear brake sensitivity should not be evident. On occasions where desired results are not achieved, burnish procedure should be repeated.

Note: A 20 ft/sec deceleration is equivalent to bringing the vehicle to a halt from 45 mph in 3.5 seconds.

3. If the desired results are not achieved by carrying out steps 1 and 2, rear braking can be further reduced by replacing the original rear wheel cylinders (P/N 18009377) on both sides with rear wheel cyliners P/N 18012303. Rear wheel cylinders must be replaced in pairs. Refer to S-Truck Service Manual X-8329, Pages 5-25, for the rear wheel, cylinder removal and installation procedure. Changing of rear wheel cylinders is not recommended for 1982/83 S/T 15 Pickups with manual brake apply systems.

Decelerometers are available from various sources, one of which is:

Ammco Tool Company 2100 Commonwealth Avenue N. Chicago, IL 60064

General Motors bulletins are intended for use by professional technicians, not a "do-it-yourselfer". They are written to inform those technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions and know-how to do a job properly and safely. If a condition is described, do not assume that the bulletin applies to your vehicle, or that your vehicle will have that condition. See a General Motors dealer servicing your brand of General Motors vehicle for information on whether your vehicle may benefit from the information.