Conditions and Causes

CONDITION

Some vehicles may exhibit a pulsating or vibrating condition (roughness) when the brakes are applied. This bulletin specifically addresses pulsation resulting from disc brake rotor thickness variation. It should be recognized that vibration perceived by the customer may be induced by rear brakes or may also be due to vibration from other components such as drivetrain, tires and/or road surface conditions.

CAUSES

One possible cause of rear drum brake pulsation is an out-of-round condition (radial runout) in a rear drum. This may occur during manufacture; however, a brake drum may also be distorted if pried off, struck with a hammer or dropped during routine brake inspection or service.

Causes of rear brake pulsation on vehicles equipped with four-wheel disc brakes are essentially the same as those described for front brake pulsation.

FRONT BRAKE PULSATIONS

One possible cause of front brake pulsation is variation in the thickness of the disc brake rotors. This condition generally appears after the vehicle has accumulated some mileage. Rotor thickness variation is the result of a combination of conditions that cause the rotor to wear while the vehicle is in motion and the brakes are not
applied. All of the following three elements must be present:

1. Rotor/Bearing System Runout
2. Caliper Drag
3. Abrasive Lining Material

FIGURE 1 - HOLE WHEEL NUT TIGHTENING SEQUENCE:

1. Rotor/Bearing System Runout

For uneven wear to occur, rotor/bearing system runout must be great enough to produce high spots on the rotor. Lateral runout on brake rotors may be caused by tolerance "stack-up" and by distortion due to over tightening wheel nuts or tightening the wheel nuts in the wrong sequence.

(To prevent distortion of the hub and rotor assembly, wheel nuts should be tightened in two steps using a suitable torque wrench and following the alternate nut tightening sequence shown in Figure 1. Applying torque in two steps helps reduce distortion by providing a gradual and even distribution of clamp load over the surface of the hub.)

FIGURE 2 - CALIPER TO BRACKET CLEARANCE:

2. Caliper Drag

Disc brake pads (inboard and/or outboard) must drag sufficiently to contact high spots on the rotor when brakes are not applied. Caliper drag is usually caused by interference between the caliper and the bracket stops on the steering knuckle or by binding of the caliper mounting bolt sleeve bushings.

(Caliper to bracket stop clearance should be checked with calipers in position and the mounting bolts torqued to specification. Clearance, measured at the two locations shown in Figure 2, should be within specifications for the vehicle tested. If clearance is too great, the caliper may move and cause a clunking noise when brakes are applied. If there is not enough clearance, the caliper may drag on the knuckle. Caliper bracket stops can be filed to adjust clearance.)

When servicing calipers, caliper mounting bolts should be cleaned and inspected for rust corrosion. Always replace corroded mounting bolts; do not attempt to polish away

FIGURE 3 - LUBRICATING CALIPER CAVITY:

corrosion. To prevent binding, the mounting bolt sleeve bushings should be coated with silicone lubricant, part number 1052863 or equivalent. Refer to Figure 3. A light application of silicone lubricant should also be applied to the caliper bracket stops.
1989 cars are equipped with a new caliper mounting bolt and sleeve bushing assembly that reduces the possibility of binding. New parts are available for service of past model front wheel drive cars as follows:

Carline Part Number

Bonneville, 6000 Heavy 18016163
Sunbird, Grand Am, 6000 Light 18016164

Parts are currently available from GMSPO.

3. Abrasive Lining Material

For uneven rotor wear to occur, shoe lining material must be abrasive enough to cause wear while rubbing against rotor high spots. Some types of brake lining material are more abrasive than others. The more abrasive the material, the more quickly wear will occur when rotor high spots contact the lining.

Product engineering selects the type of brake lining material to be used based on the friction coefficient requirements of a particular vehicle. Brake linings and other components should be replaced only with the parts recommended in the parts catalog or the latest service bulletin.