Posting on the diesel discussion will get you more help. There are vacuum diagrams posted as stickys on the diesel discussion. All the vacuum systems are interconnected on your car. A leak in any system will cause shut off problems typically.]
Using the vacuum diagrams and a Mity Vac you should be able to track down the leak. Typically the vacuum pods in either the climate control system or door locks develop leaks. The shut off valve can also tear its diaphragm.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13