Why do my rear brakes lock up?

Dear Car Talk

Dear Car Talk | Oct 01, 1989

Dear Tom and Ray:

I have a 1977 Olds Delta 88 that has a few problems. The one that bothers me the most is that the few times it has been necessary to quickly apply the brakes, the rear wheels lock, and the car skids. The brakes were replaced, the master cylinder was replaced, and the drums were turned in an attempt to fix this, but the brakes have locked again since these repairs were made. What else can I check?

TOM: There are several mechanical parts you can check. One of the wheel cylinders or rear axle seals could be leaking, and the leaking oil (especially if it's a small amount) could cause a wheel to lock up. If that isn't what's wrong, you might replace the proportioning valve. That valve is supposed to send the proper amount of hydrolic braking pressure to the front and rear wheels.

RAY: You might also check some simple things that mechanics often overlook in a situation like this. Incorrect tire pressure--either too much or too little--can cause a tire to lose traction, and that can make it lock up. Lousy tire tread can also cause lock up during hard braking. Finally, a bad rear shock absorber might cause wheel hop, which could let a wheel lock when it's in the air, and stay locked when it returns to the pavement.

TOM: Keep working on this, Harry, it's really quite dangerous. Start with the easy stuff. You can play with the tire pressure yourself. Who knows, perhaps you can get the front wheels to do this too. Just kidding!

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