I had same problem with my 83D. Took it to my trusty trans man and he fixed it. Said it was a "low gear piston". To this day I have now idea if such a part exists, but he fixed it.. circa $150, I think.

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Nello Tare

1982 300D (266,001 mi.) looking and running great

Previously owned:
1976 450 SLC
1983 300D
1976 300D
also
1982 Jaguar XJ6 (loved, but gone)