I've got no clear idea on what is causing the problem, but if I were you, I'd run a line into the passenger compartment so I could monitor the vacuum as I'm driving and figure out when and how the vacuum is disappearing. It might be relevant if it is disappearing randomly or only when you step on the brakes.

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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