Do search on 'center vents' and you'll see a lot of tips on how to test it. Listen to it to see if you canhear a vacuum when it is supposed to open. Also use a Might Vac to pull a vacuum on the actuator and line to see if and where it leaks.

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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