On the line from the ignition switch to the shutoff valve you should have 0" of vac when the car is running, and when you shut off the car it should jump up to around 15" of vac and then bleed back down to 0" quickly after the engine is shut down.
It sounds like the shut-off valve on the ignition is bad.
Here is the shut-off valve on the ignition switch. -
http://catalog.worldpac.com/mercedesshop/sophio/quote.jsp?clientid=catalog.mercedesshop&cookieid=1L00T87R028I0OQRRI&baseurl=http://catalog.peachparts.com/&partner=mercedesshop&year=1979&product=D2020-11741&application=000015943