I have the same problem. Brakes are sometimes weak, parallel parking can get a bit scary. Sometimes engine doesn't shut off. The Mrs hates much about the car, but these are probably her biggest issues (aside from the water leaks, manual glowing on a switch, rough idle, leaking fuel tank, oil leaks...)

I figured it was a vacuum problem, and I've bought a hand pump tester on Ebay, but haven't gotten around to do it just yet.

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1985 W460 300GD OM617.912 w/ STT turbo (ongoing OM606 engine swap)
2011 W212 E220 CDI