I believe if you operate the controls and listen to the servo, you should be able to hear the valve operating.
You probably don't want to buy a new one. Close to $1000. A rebuilt with a metal body is the best bet. Randy at European Parts Specialists had the best price last time I looked. Around $300. Do a search on this forum and his phone # should come up.
I'd shy away from a used one unless it is a metal bodied one. Those things are prone to cracking.
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