Another thing to keep in mind while following the previously posted good advice is that the vacuum check valves tend to get old and begin failing by not "checking", or in other words they allow air in both directions. I had this problem on my '79 450SL, several check valves just didn't work.

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C