That sounds like one of the pistons in the rear calipers got stuck.

You should remove the wheel and caliper on the affected wheel and verify operation of both pistons on the caliper. They should move in very easy in by squeezing them. Using a C clamp you can secure one side and check the movement of the other by lightly depressing the brake pedal. Then vice-versa for the other side.

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AJ

1985 300D (SOLD)