You don't say which car, but the intermittent nature of it tells me the odds are you just need to buy this part:

http://catalog.worldpac.com/mercedesshop/sophio/wizard.jsp?partner=mercedesshop&clientid=catalog.mercedesshop&baseurl=http://catalog.peachparts.com/&cookieid=26W03483526W034VKV&year=1983&make=MB&model=300-DT-001&category=All&part=Vacuum+Pump+Check+Valve

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The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)