It's all vacuum! It could be the 3/2 way valve on to of the motor or any number of possible vacuum leaks. Not a huge problem, just hard to pinpoint. I have replaced almost all vacuum lines on my car as a precaution/hobby with good woven vacuum hose from the MB parts dealer. $3.50/ft is money well spent. $7.00 so far...as far as vacuum pump??????

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1982 300D Turbo "Helga"
380,000 miles