A 1992 Buick Skylark with a right rear brake lockup problem

Dear Troubleshooters: I am a new mechanic at the Goodyear ASC store in Watertown, and a 1992 Buick Skylark has me stumped. It has a right rear brake lockup problem that I can't pinpoint. I replaced the wheel cylinders and master cylinder proportioning valve; flushed and rebled the system; and installed new drums, but it still locks up. I did not replace the backing plates, but I am not sure where to go from here.Any suggestions would be great. Thanks in advance. Joseph BoscoeGoodyear Auto Service CenterWatertown, NY

Dear Mr. Boscoe: First of all, does this vehicle have ABS? Is the ABS warning light working and are there any codes in memory? Check this first before proceeding.

Drum brake lockup can result from many different conditions. These can be broken down into either hydraulic or mechanical problems. It appears that you have covered most of the hydraulic concerns.

GM used several different size bore diameters in their wheel cylinders back then. Are you sure the two rear cylinders are matched? Also check the rear brake hoses. A hose that is deteriorated internally could allow fluid pressure to pass but not immediately return.

Inspect the backing plate shoe contact surfaces for grooves that may restrict shoe movement. If grooves cannot be removed by light sanding, replace the backing plate.

Check the parking brake cable adjustment. A cable that is adjusted with insufficient free-play or a frozen cable will allow the brake shoes to grab prematurely.

Be sure all brake hardware such as springs and hold-down pins are in good condition. Look for any leakage of oil or fluid, which can contaminate the brake linings. Check to be sure the shoes are installed correctly and the lining to drum clearance is properly adjusted.

Good luck in your new venture.