You wont get 20 inches at your elevation. 18 is more like the best you'll get. But as Zero said, it's a vacuum problem. Given the fact that you don't mention not having power brakes, I'll assume the vacuum pump is working. I'm guessing the most likely source of the problem is that the vacuum line to the shut off valve is disconnected. Brown line, going down to shut off valve onthe back of the IP between the IP and oil filter housing.

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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