I have a brake problem that seems to be getting dangerous. I drive a 1989 Mercury Cougar, and sometimes when I hit the brakes hard the rear of the car skids sideways. The brakes are fairly new and the repair shop that installed them can't find a problem. Any ideas would be helpful.
Your brakes require a thorough inspection, and your repair shop must be missing something.
This is a brake equalization scenario. With any brake system it's crucial that each wheel is being slowed at a rate very close to the others, especially the one on the other side of the car.
If the braking force is not equal in the front brakes, typically, a "brake pull" will be the primary symptom. A brake pull means the vehicle dips and turns to one side while applying the brakes.
When the equalization problem exists in the rear brakes, you really can't feel the pull in the early stages. In these cases, like your situation, the problem may not be noticed until one of the rear wheels locks up causing the back of the car to slide out from behind you. Not good.
Let's get down to the possible causes. First we eliminate the basics, like tire condition and air pressure along with going over the entire suspension. When dealing with disc brake applications, choices would be a frozen caliper, obstructed brake lines/hoses or problems with the brake pads and/or rotors themselves (cracks, defects, contamination or simply worn-out).
We're now zooming in on your Cougar's problem with drum brakes. Chances are that your mechanic did a quick inspection of the rears and didn't see any contamination of the brake shoes from a leaking axle seal or wheel cylinder. Some items they might have overlooked are a wheel cylinder freezing up (just like a caliper), a drum that has been machined too thin, or the brake shoes were installed incorrectly. One shoe is actually bigger than the other, and if you don't install the smaller one (primary) toward the front, this will affect brake equalization.