20" of vacuum is fine. Leak could be anywhere so I wouldn't condemn the booster without testing it independently which you can do with the MityVac.

What year is the car? Does it have the loop plugs or pencil plugs? Loop plugs have heavy squiggly resistor wires that go from plug to plug with the front plug grounded to the block.

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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