BRAKE EFFECTIVENESS CHANGE AFTER ROTOR & BRAKE PAD WORK

SUBJECT: CHANGE IN BRAKE EFFECTIVENESS AFTER DISC BRAKE ROTOR REFINISHING/PAD REPLACEMENT

MODELS AFFECTED: ALL YEARS/MODELS WITH DISC BRAKES

This bulletin has been revised to add information about hub and rotor cleaning, proper torquing technique, revise machining table, and to change the specification for maximum scoring depth.

Some comments have been received about a change in perceived braking effectiveness which occurs after brake rotors have been refinished and/or the disc brake pads have been replaced. New lining materials have been formulated for increased lining life and to reduce brake squeal. Also, Federal regulations currently prohibit the use of asbestos in Original Equipment Manufactured (O.E.M.) front disc brake linings, and totally ban asbestos from all O.E.M. brake linings in the near future. Due to these changes, initial rotor surface finish is more critical than in the past and is required for good brake performance.

Following are some recommended actions/procedures to assure proper performance of the brake systems on all vehicles after rotor and/or pad service has been performed:

1. When performing routine brake maintenance, such as replacing worn disc brake pads or shoes, DO NOT refinish the disc brake rotors or drums unless:

A. There is a brake pulsation condition present, and this pulsation is found to be caused by the brake rotors or drums, or

B. The rotors and/or drums are excessively scored. Surface scoring that does not exceed 1.2mm (0.050 in.) on rotors or drums should not affect brake operation.

Before removing rotors from the hub assembly, mark the rotor and wheel stud so that the rotor is re-installed in the same position.

If the rotors are removed, it is very important that rust and scale be removed from the rotor and hub mating surfaces. Failure to do so may introduce excessive lateral runout when the rotor is mounted on the brake lathe, or when the rotor is re-installed on the hub.

When refinishing disc brake rotors, it is important that the brake lathe be in good operating condition and that all tool bits are sharp. Recommended vibration dampeners and/or adaptors should be used and should be clean and free of nicks (remember, 1988-91 Regal models require the use of adaptor J-37160 because of the modular design). The following table shows the recommended procedure for rotor machining when using a benchtype brake lathe:

IMPORTANT: In cases where rotors are being refinished to correct a problem with brake pulsation, it is recommended that an On-vehicle Brake Lathe (VBG B20/366 - GM Equipment or equivalent - Kent Moore J-37704), be used to achieve the best results.

ROUGH CUT FINISH CUT --------- ----------- Spindle Speed 150 RPM 150 RPM Depth of Cut (per side) 0.127mm (0.005") 0.051mm (0.002") Tool Cross Feed per 0.152mm - 0.254mm 0.051mm (0.002") Max Revolution (0.006" - 0.010") Vibration Damper Yes Yes Sand Rotors-Final Finish No Yes

It is important that a rough cut and a finish cut be made. All brake lathes use a single-point cutting tool which is not capable of giving the necessary surface finish. A SECONDARY FINISHING OPERATION MUST BE PERFORMED TO OBTAIN THE NECESSARY SURFACE FINISH. When using an on-brake lathe, the tool vendor's recommendations should be followed.

An acceptable finish can be obtained using the Ammco Model 8350 Safe Swirl Disc Rotor Grinder, or equivalent, using 120 grit sandpaper and sanding each rotor surface with moderate pressure for a minimum of 60 seconds with the rotor turning at 150 RPM. An alternate method is to use a sanding block with 150 grit aluminum oxide sandpaper i.e. 3M "A" weight paper No. 02446 or equivalent.

After the rotor has been sanded, the surfaces must be cleaned with a solvent such as GM Brakes Parts Cleaner #12345754, denatured alcohol, or equivalent.

THE FINISHED ROTOR SURFACE SHOULD BE AS CLOSE TO THAT OF A NEW ROTOR AS POSSIBLE. FAILURE TO OBTAIN THE BEST POSSIBLE ROTOR FINISH WILL AFFECT INITIAL BRAKING PERFORMANCE.

CAUTION:

ROTORS OR DRUMS SHOULD ALWAYS BE REPLACED IF TURNING WILL RESULT IN A ROTOR OR DRUM THAT DOES NOT MEET MANUFACTURER SPECIFICATIONS FOR MINIMUM ROTOR THICKNESS OR MAXIMUM DRUM DIAMETER.

NOTICE: When re-installing the tire and wheel assemblies, it is very important that proper procedures be followed when installing and torquing the wheel nuts:

A. Finger start all wheel nuts.

B. Tighten wheel nuts to specified torque (use the "star", or alternating nut pattern) using a torque wrench.

DO NOT USE AN IMPACT WRENCH. UNEVEN AND/OR EXCESSIVE TORQUING OF THE WHEEL NUTS HAS BEEN FOUND TO DISTORT ROTORS, RESULTING IN PREMATURE CUSTOMER COMEBACKS FOR BRAKE PULSATION.

3. After the brake pads have been replaced and/or the rotors have been refinished, it is recommended that the new braking surfaces be broken in, or burnished, to properly seat them. This can be accomplished by making 20 stops from 30 mph, using medium to firm pressure. Care should be taken to avoid overheating the brakes.

4. It is strongly recommended that the correct, specified General Motors replacement part(s) be used when servicing G.M. vehicles. General Motors does not test non-G.M. parts for proper performance on G.M. vehicles. Therefore, the use of non-G.M. parts may result in unacceptable vehicle performance. It is also important that the correct G.M. part(s) be used in the correct G.M. application.

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.