1998 Chevrolet Cavalier

I have a 98 cavalier 4 door with 180,000 miles with a 2.2 L engine. It seems the back brake on one side will lock up at times and other times all is fine. Other times the car will seem to squat at the back end very hard just when we are almost at a stop at a light. We have taken it to the dealer and had the brakes all replaced and still have the problem. We then took it to a great mechanic who does everything else for us who can't find the problem. Any ideas? Does it have a proportioning valve or something that may be bad?
November 19, 2007.

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