I believe the shutoff line is brown with a blue stripe, supply is plain brown. You need to check ALL the vac lines and connetors in the engine compartment, they are all getting pretty hard by now.
Other things that leak will cause the vac supply to be low, so what I would do is hook up the mitivac to the shutoff line and pull the key. You must get decent vac there. If you don't, pull off the supply line somewhere in the engine compartment and see if you have vac on the supply side (should be 11" at least). If not, check the fitting for cracks or crap clogging the hole in the main vac line fitting.
Oil in the keyswitch means someone has already replaced the servo. See if it holds vac with the mitiVac (pump down and watch the gauge -- if it doesn't drop off, the servo is fine). Next, with key out, reconnect the vac line to the servo and pump down the brown line. If it won't hold vac, and the rubber connectors are OK, the keyswitch has been ruined by the oil.
Check to make sure the hoses are on the switch are on corretly,, too -- if they are backwards, it leaks.
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!