Problem: Rear brake sensitivity, lock-up and hydraulic failure
Cause: Clip design of certain GM wheel cylinders allows for the wheel cylinder to rotate. This rotation can cause rear brake sensitivity problems and eventually allow the wheel cylinder to rotate to a point where the piston is no longer against the brake shoe. This allows the piston to be pushed out of the wheel cylinder bore which causes hydraulic failure.
Solution: Checking the GM Clip In Style Wheel Cylinder
It is critical to check for excessive looseness in this type of wheel cylinder/ backing plate arrangement. Follow the procedures below to check the GM clip in style wheel cylinder:
1. Start with a visual inspection of the wheel cylinder. It should be in a horizontal position in reference to the backing plate. If the wheel cylinder is at an angle the backing plate is probably worn.
2. Even if the wheel cylinder is properly aligned (horizontal) perform this check. Attempt to rotate the wheel cylinder in the backing plate. Only the slightest movement should be felt (If any). If the wheel cylinder rocks in the backing plate excessive wear has taken place and the wheel cylinder and backing plate should be serviced.
NOTE: Failure to service the wheel cylinder and backing plate when this condition is present can lead to rear brake failure.
Figure 63.1 
Figure 63.2Most wheel cylinders are bolted to the backing plate. GM has used a different method on a number of their vehicles since the late 70’s. Instead of bolting the wheel cylinder to the backing plate they use a retaining clip as shown in Figure 63.1. The backing plate and wheel cylinder are shaped to fit together in a way that is supposed to prevent the wheel cylinder from rotating. The backing plate is shaped like that shown in Figure 63.2. This method works adequately as long as the backing plate and wheel cylinder are in good shape. As corrosion takes place the hole in the backing plate enlarges as shown in Figure 63.3 and the wheel cylinder casting deteriorates. This allows the wheel cylinder to move. The amount of movement will increase over time. If left unchecked the wheel cylinder can rotate to a point where the wheel cylinder piston no longer contacts the brake shoe. If this happens the wheel cylinder piston will be forced out of its bore when the brakes are applied. This condition usually always presents itself on the primary piston due to the duo-servo design of the brakes. If the wheel cylinder piston “blows” out rear brake function will be lost. Aftermarket wheel cylinders have been designed with an extension to the casting that helps prevent this from happening (See Figure 63.4)
Figure 63.3 
Figure 63.4
