You have classic "old benz" vacuum issues. The door actuators have four diaphrams each (none in the drivers's door) that crack and leak, the rubber connectors on the hard plastic lines leak, and the plastic lines in the engine compartment can get brittle from head and hydrocarbon exposure.

What I would do is check each system for vacuum integrity. There are four lines that connect the the main vac line from the vac pump to the brake booster -- brown in the shutoff, white is engine/tranny/EGR supply, yellow is door locks, and green is climate control. Disconnect one at a time and apply vac to the line. Door locks and climate control have reservoirs, so it will take many pumps of the hand pump to pull a vac, but it should so accumulation of vac as you go. If not, it's got a fairly large leak. You will have to trace them down one at a time.

For the shutoff, check the servo itself, the diaphram eventually goes. If it's bad, it will pull engine oil into the keyswitch and that will need to be replaced, so don't mess about too long.

New door lock diaphrams are available for $5 or so each from several places, it's worth replacing them all.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!