Make sure the booster vacuum line doesn't have a leak in it:
http://catalog.peachparts.com/ShopByVehicle.epc?q=1981-Mb-300sd-Brake&yearid=1981%40%401981&makeid=MB%40%40MB%40%40X&modelid=300%2DSD%2D002%40%40300SD&catid=N%40%40Brake&subcatid=N4020@@Booster+Vacuum+Hose&mode=PA

Do you have any other vac issues other than the shutoff? Like door lock problems?

__________________
Chad
2013 Jeep Unlimited Rubicon
OBK#44
"Pleasure in the job puts perfection in the work." - Aristotle (384-322 B.C.)

SOLD
1985 300TD - Red Dragon
1986 300SDL - Coda
1991 - 300TE
1995 - E320
1985 300CD - Gladys
2006 Nissan Pathfinder LE
1998 Acura 3.0 CL