Front Brake Pulsations

FIGURE 1 - HOLE WHEEL NUT TIGHTENING SEQUENCE:

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 Run Out
2. Caliper Drag
3. Abrasive Lining Material

1. Rotor/Bearing System Run Out - For uneven wear to occur, rotor/bearing system run out must be great enough to produce high spots on the rotor. Lateral run out 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 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:

Figure 3 - Lubricating Caliper Cavity:

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 caliber 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 corrosion. Always replace corroded mounting bolts; do not attempt to polish away corrosion. To prevent binding, the mounting bolt sleeve bushings should be coated with silicone lubricant, P/N 12345579 or equivalent. Refer to Figure 3. A light application of silicone lubricant should also be applied to the caliper bracket stops.

1989 DeVilles and Fleetwoods 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 1982-1988 Cimarrons, 1985-1988 DeVilles, and Fleetwoods as follows:

Carline PN

DeVille, Fleetwood 18016163
Cimarron 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 GMSPO parts catalog or the latest service bulletin.