my 83 300d did the same thing and it was the b2 piston.

round cover on the passenger side.

You have to remove the cross member and lower the tranny as far as possible then push in the round cover remove the retaining ring and cover and the piston should pop out. ( Mine fell out in pieces).

Put in new one, install new oring cover seal, install cover and retainer ring cross member and your done.

Do a search and you will find detailed instructions.

Steve

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85 300d in progress