Does sound like there's a problem with the VCV. The vacuum should bleed down as the rack is opened. Have you hooked up the MityVac and run it out onto the cowl and taken readings as you drive around? I'm pretty sure I remember a thread on here where someone opened one up and tried to fix it.

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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