I've always had a soft spot in my heart for vacuum gauges since the one I had in my 69 Ford van allowed me to diagnose a bent pushrod in about 10 seconds (the gauge was fluctuating since that cylinder wasn't pulling vacuum anymore). I think I'd want it tied into the main vacuum line to monitor the overall state of the vacuum system. It could give you early warning that your brakes are going to be hard.

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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