Started my daily driver '85 CA-spec 300D the other day and, sitting on a moderate downhill, it immediately began gently surging backward against park. Reverse engaged fine, neutral mimics park in the steady surging backward, D or any of the other forward ranges stops the surging but it does not want to move forward and I wasn't going to give it much gas for fear of becoming the referee in this apparent fight between D and R. Even after shutting the car off, in neutral it's not completely locked up but does not want to freely roll downhill.

Having been into a 722.5 that lost reverse before, I know how the B3 piston and big round plate with all the springs to release it works. Is there a possible hydraulic (aka valve body) issue that could cause this half-engaging reverse in all other gears? Or did the spring plate or snap ring for B3 piston bite the dust.

226K on the car, very good shift behavior and regular fluid changes the last 80K I've owned it.

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'82 300CD
"Pearl", the very first turbo diesel 123 coupe
Totaled 11/23/18, rebuild in progress.
'85 300TD, "Artemis".
'78 300D euro, "Ol' Red", mostly retired.
'85 300D, "Gandalf".