The only successful way to troubleshoot a vacuum system is to use a tool, like the harbor freight item 92474-0VGA, a low cost vacuum pump with gauge. ($16.99), or a mity-vac. You cannot successfully test the system with a "smoke test".

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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