Successfully completed the rebuild of the vac pump (single line, piston style) on my 617.952 two weeks ago. Worked beautifully until yesterday when the self-locking nut that holds the piston on its shaft decided it no longer wanted to stay where it should and blew a 3/4" hole through the center of the vac pump face plate. I found the center of the plate sitting on the core support but the nut was long gone. It scored the threads on the shaft a little as well as the center of the piston (by the hole) before its escape. It may have been my fault but I suspect it was a faulty nut- no way of knowing now. In any case, make sure you triple check this nut (and everything else) before/during re-assembly!
My indie friend had a spare pump (housing, and its insides, etc. $175), luckily, and I will be installing that tomorrow. Replacing the whole housing assembly with a known good one (instead of just a new nut and faceplate) to be safe in case I pull this pump and find more damage...
I really wish these things could happen during warmer weather...

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1969 220D "Berta" RIP
1982 300D turbo 250K