BRAKE EFFECTIVENESS AFTER ROTOR REFINISHING

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

VEHICLES AFFECTED: ALL YEARS/MODELS W/DISC BRAKES

Some comments have been received about a change in perceived braking effectiveness which occurs after rotors have been refinished and/or disc brake pads have been replaced. Most of these comments have been on W models, with some comments also received on E, C and H models.

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 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. Light surface scoring that does not exceed 0.4mm (0.015") on rotors or 0.5mm (0.020") on drums should not affect brake operation.

2. When refinishing disc brake rotors, it is important that the brake lathe be in good operating condition and that all tools or bits are sharp. Recommended vibration dampeners and/or adaptors should be used and should be clean and free of nicks (remember, 1988-90 W models require the use of an adaptor, J-37160, because of the two-piece design). The following table shows the recommended procedure for rotor machining:

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 Rev 0.152mm - 0.254mm 0.051mm (0.002") Max (0.006" - 0.010") Vibration Damper Yes Yes Swirl Pattern-120 Grit Yes Yes

It is VERY IMPORTANT that the rotor surface finish be made nondirectional by dressing the rotor surfaces with a sanding disc power tool such as Ammco model 8350 Safe Swirl Disc Rotor Grinder, or equivalent, using 120 grit. The finished rotor surface should be as close to that of a new rotor as possible. Failure to obtain the best possible rotor finish may affect initial braking performance.

3. After brake pads have been replaced and/or 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 50 km/m (30 mph), using medium to firm pressure. Take care 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. For example, some 'A' model disc brake pads ('A' Heavy) will fit on 'C and H' models, but will not provide the same braking performance as pads specified for use on C and H vehicles. It may seem preferable to stock fewer brake pad start numbers, but customer dissatisfaction may result if vehicle performance is affected.

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.