Update on the problem:
I tested the vacuum at the switchover valve. It is fine and pulls about 10 inches of vacuum. When I hook the line up to the center vent and T into it to test the vacuum, I can pull a vacuum and open the center vents. However, when the ACC operates, what happens is that the vacuum increases up the word 'Zero' on the scale on the Mighty VAc. When it reaches the 'o' on zero it stops and does not open the vents. If I give it a few pumps on the might vac up to 1 or 2, then it increases up to 10 on its own and opens the vents. If I release that vacuum thru the mighty vac back down to 1 it will build up again and work fine. If I hold the release until it goes all the way back to 'z' on zero and hold it there for a second, the same problem arises. It will only go back up to 'o' on zero until I give it some help with a couple of pumps.
If I disconnect the line from the actuator and test the vacuum, it is working fine.
Anyone have any clue as to what the problem is?
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