I suggest replacing both rear calipers, flex hoses, pads and rotors.

Best guess:
Collapsed flex hoses acting as one way valves and/or both calipers have sticking pistons + the rotors are warped past salvage.

Brake:
http://www.peachparts.com/shopforum/showthread.php?t=142411


Have a great day.

__________________
ASE Master Mechanic

Prototype R&D/testing:
Thermal & Aerodynamic System Engineering (TASE) Senior vehicle instrumentation technician.
Noise Vibration and Harshness (NVH).
Dynamometer.
Heat exchanger durability.
HV-A/C Climate Control.
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1985 300SD
1983 300D

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