Since you've got an '88 300E, You might consider adjusting the shoes. Although Mike's advice is good, you should really find out how much you've got left in the parking brake shoes. To adjust, remove rear wheels, use a flat bladed screwdriver in one of the lug bolt holes (use a flashlight to line the hole up with the adjusting wheel), turn the wheel until the shoes just start to contact the inside of the rotor hub and back off until they rotate freely. Then the brakes should cinch up solid within 7-8 clicks. If not, then..adjust the slack in the cables. if the shoes are worn enough, there may not be enough adjustment in the cables to get the brake to set properly.

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Jeff Lawrence
1987 300e
1989 300e
1987 BMW 325