Have you checked the switchover valve? I assume you have checked to make sure there is no vacuum getting to the servo when the pushbutton unit is the appropriate position. The pushbutton unit activates a small valve behind the unit which sends the vacuum to the servos. They are electrically operated and fairly inexpensive.
You have to remove the pushbutton unit and look just up and behind the pushbuttons.

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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