BRAKE BALANCE PROCEDURE REAR WHEEL SLIDE (VAC. P/B). GMC Truck S15 Pickup

VEHICLES AFFECTED: 1982-1983 S/T Pickups And Jimmy/Blazer With Vacuum Power (RPO J50) --------------------------------------

S/T Pickup Trucks and Blazer/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 is consistent with industry practices 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 alide is desirable to maintain steering and shorten stopping distance during braking.

Some customers who operate their S/T Pickup or Blazer/Jimmys lightly loaded most of the time may prefer less rear braking. If so, the following procedure is an acceptable metnod of reducing rear braking on 1982-1983 S/T Pickups and Blazer/Jimmys with vacuum power brakes. It will result in a higher rate of deceleration at wheel alide 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 Pickups and Blazer/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 originel 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 Product Service Bulletin #305, Group 50, October 31, 1977.

Following is the information from that Bulletin.

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.

- Warmup

Using the parking brake only, make 12 stops from 40 mph at a deceleration of 6 ft./sec2 at 0.2 mile intervals.

NOTE: A 6 ft./sec2 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 ton stope from 30 mph at a decoleration of 6 ft./sec2 at 0.5 mile intervals.

NOTE: A 6 ft./sec2 decelaration 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 to 20 ft./sec2 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./sec2 deceleration in 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 cylinders 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 1982-1983 S/T 15 Pickups with manual brake apply systems.

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

Moto-Rite Ltd. 100 Norfinch Drive Downsview, Ontario M3N 1X2

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.