This car has small brake shoes inside the rear rotor that are the parking brake assembly. Thats where I would look first. Could be worn down to metal on metal. Also I've seen wrong lug bolts used which were too long that can touch the shoes and can cause binding and noise. I presume you know how to remove a rear caliper and slide the rear rotors off? If the rotor wont slide off, you can rotate it and thru one of the stud holes, locate the star adjuster to adjust in the shoes. The shoes and hardware are just smaller versions of 70's era drum brake cars.

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81 300CD (sold) 1972 280 SEL 4.5 (sold) 1966 250 S 4 spd (sold) 1974 450 SL (sold) 86 BMW 325ES (sold), 1973 280C (sold) 1988 300 SE.