Sounds just like the B2 piston. There is cheaper places but I got mine from the dealer. It is a good time to do K1 as you will have the pan off and fluid drained already.

You will need B2 piston, bushing, and seal. Fastlane has good prices on these parts and supports this forum.

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81 300SD daily driver/project
86 420SEL (sold)
85 380SE(in the graveyard)