If you have a large leak upstream, it may affect the reading you are getting. You have not yet determined that your vacuum pump is not working properly. Disconnect all vacuum connections from the main hose and plug them (including the brake booster.) Then attach the vacuum gauge to one point. Assumming no leaks in the main hose and all plugged properly, then you are testing/isolating the vacuum pump.

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2009 ML320 Bluetec
1985 300CD
1981 300TD

Past Mercedes
1979 300TD
1982 300TD
2000 E320 4Matic Wagon
1998 E430
1984 300SD
1980 300SD