BRAKE PERFORMANCE INCREASED REAR BRAKING REVISED

Model and Year: 1985-87 M-VANS

This bulletin cancels and replaces Chevrolet Dealer Service Bulletin 86-T-219 (Section 5) dated February, 1987, and includes additional diagnostic information. All copies of 86-T-219 should be discarded.

Astro vans are designed to carry varying loads, and therefore, they are equipped with brakes that are capable of good stopping performance under a variety of conditions. Some customers who operate their Astro vans heavily loaded most of the time may prefer additional rear braking. If so, the following procedure is an acceptable method of increasing rear braking on 1985, 1986 and some 1987 Astro vans. It will result in a higher rate of deceleration at wheel slide and shorter stopping distances in a heavily loaded condition. The customer should be informed that there will be an increase in rear brake performance and the potential for rear wheel skid in unladen applications.

Astro vans, in both their original condition or as changed by this bulletin, meet all the stopping requirements of Federal Motor Vehicle Safety Standard 105 for Light Duty Trucks.

Service Procedure

Note: Care must be taken to prevent brake fluid from contacting any painted surface.

1985-1986 Models:

Always ensure that the brake system is operational after performing brake work and before returning the vehicle to the customer.

Steps A, B, and D involve all subject vehicles. Step C is engine dependent.

A. Replacement of the combination valve, Figure 1.

1. Disconnect negative battery cable.

2. Disconnect the four brake lines at the combination valve.

3. Disconnect electrical harness at the combination valve.

4. Disconnect combination valve at bracket on the brake booster and discard valve.

5. Install new combination valve (P/N 15571205).

6. Reverse above procedure for reinstallation and tighten bracket nuts to 50 Nm (37 lbs ft).

7. Reinstall brake lines and torque nuts to 25 Nm (18 lbs ft).

B. Replacement of the brake shoes, wheel cylinders, and link pins, Figure 2.

1. Loosen lug nuts on rear wheels.

2. Hoist vehicle and remove lug nuts and rear wheels.

3. Remove brake drum retaining clips (2).

4. Remove brake drum.

5. Remove brake shoe hold down and retainer springs.

6. Remove brake shoes and discard.

7. Disconnect brake line at wheel cylinder.

8. Remove wheel cylinder and discard.

9. Install new wheel cylinder (P/N 18004888) after inserting link pins (2) (P/N 5464748).

10. Tighten wheel cylinder bolts to 18 Nm (13 lbs ft).

11. Connect brake line to cylinder, torque nut to 20 Nm (14.5 lbs ft).

12. Install new brake shoes (P/N 12321427).

13. Adjust running clearance to .030 +/- .005" adjustment clearance.

14. Reinstall brake drum, retainer clips, and wheel.

15. Bleed brakes, refer to Astro Service manual, pages 5-5 and 5-6.

16. Repeat on opposite wheel.

1985 Model 4.3L Carbureted Engine --------------------------------- C. Reduce cold idle speed to 1600 rpm.

1. Remove dash extension.

2. Remove engine cover.

3. Install tachometer. The tachometer (tach) terminal is next to the engine control switch (bat) connector on the distributor cap (Reference Figure 3).

Note: The tachometer terminal must never be allowed to touch ground because damage to the module inside the distributor and/or ignition coil can result.

4. Reduce cold idle speed to 1600 rpm by adjusting the fast idle screw. Refer to tune up label for procedure.

5. Remove tachometer and reinstall engine cover and dash extension.

6. On a clean dry surface, next to the existing tune up label located on the fan shroud, place label P/N 10066488

1986 Model 4.3L TBI Engines ---------------------------- C. Replacement of the PROM, Figure 4.

1986 model vehicles built before VINS listed below require PROM replacement.

VIN EMISSIONS

GB203188 Federal Emissions GB203623 california Emissions

Vehicles built after these VIN breakpoints have PROM factory installed.

Note: Any time the PROM is installed backwards and the ignition switch turned on, the PROM is destroyed and must be replaced.

1. Disconnect negative battery cable.

2. Remove kick panel from right side of the vehicle (1 screw) interior.

3. Remove ECM (2 screws).

4. Disconnect electrical harnesses from ECM module.

5. Remove ECM cover (2 screws).

6. Remove PROM from ECM and replace with new PROM, P/N 16068877 (Federal), P/N 16068926 (California).

7. Replace ECM cover.

8. Reconnect electrical harness.

9. Secure ECM module to its mounting location.

10. Replace kick panel.

1985 - 1986 2.5L TBI Engine ---------------------------

C. Vehicles equipped with this engine do not require PROM replacements.

D. Brake Burnish Procedure

On a dry clean level roadway, make twelve stops using the following procedure:

First Stop: Stop vehicle from 40 mph by applying the brakes to achieve a deceleration rate of 10 ft/sec/sec. (A 10 ft/sec/sec deceleration is equivalent to stopping the vehicle from 40 mph in 6 seconds.)

Second through Eleventh Stops: Stop vehicle from 50 mph by applying the brakes to achieve a deceleration rate of 15 ft/sec/sec. (A 15 ft/sec/sec deceleration is equivalent to stopping the vehicle from 50 mph in 5 seconds.)

Twelfth Stop: Stop vehicle from 40 mph by applying the brakes to achieve a deceleration rate of 10 ft/sec/sec.

In order to allow brake temperatures to stabilize, it will be necessary to drive 1/2 mile between stops without applying brakes. Following completion of burnish procedure (twelfth stop), drive vehicle 3 miles keeping brake usage to a minimum in order to achieve sufficient brake cool down.

1987 MODELS

Effective 1987 start of production, all Astro models were manufactured with new wheel cylinders described earlier in this bulletin; however, the new combination valve was not installed in the first several months of production. The new combination valve was installed effective with the following Vehicle Identification Number:

VIN HB137507

1987 vehicles produced prior to this VIN breakpoint can be modified by following the service procedure listed below:

SERVICE PROCEDURE

1987 Model 2.5L TBI and 4.3L TBI Engines ----------------------------------------

Note: Care must be taken to prevent brake fluid from contacting any painted surface.

Always ensure that the brake system is operational after performing brake work and before returning the vehicle to the customer.

Replacement of the combination valve, Figure No. 1.

1. Disconnect negative battery cable.

2. Disconnect the four brake lines at the combination valve.

3. Disconnect electrical harness at the combination valve.

4. Disconnect combination valve at bracket on the brake booster and discard valve.

5. Install new combination valve (P/N 15571205).

6. Reverse above procedure for reinstallation and tighten bracket nuts to 50 Nm (37 lbs ft).

7. Reinstall brake lines and torque nuts to 25 Nm (18 lbs ft).

Parts List

1. Combination Valve (1) P/N 15571205 2. Wheel Cylinders (2) P/N 18004888 3. Brake Shoe Kit (1) P/N 12321427 4. Link Pin Kit (2) P/N 5464748 5. Brake Fluid DOT 3 P/N 1052535 (as required) or equivalent 6. Label (1985 4.3L Engine) (1) P/N 10066488 7. PROM for 1986 4.3L P/N 16068877 Federal engine 8. PROM for 1986 4.3L P/N 16068926 California engine

Parts are currently available from GMSPO.

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.