First, keep in mind that if the car pulls to the right, then the left rear brake may be the problem, or if it pulls left it could be the right rear brake. Also remember that it may not be a problem with one brake not working, but another working too good.
A trick I have been using for years is find a dirt road, hard dirt field, grass field, etc. Then do a semi-panic stop on one of these serfaces. After you stop, get out and look at the tire tracks left in the dirt. An outside observer also helps. Look for signs of brakes not enguaging at the same time, or not at all, or possibly one brake locked up sooner than the rest of them.
I think you get the idea. When you find one brake not working like the others, you know where to start.

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84 500 SEL (307,xxx miles)