Weak or sporadic reverse? Trans-brake not holding? Could be that the Rear Servo Piston is the culprit!
One of the often overlooked- and affordable- parts when rebuilding a C4 is the Rear Servo Piston.
If the black rubber bonded ring on your piston is hard or crumbling, its time to replace it.
Lubricate the new seal with Trans Gel or ATF before install.