Get the Mityvac WITH a vacuum guage. Or an equivilant. It does not have to be a Mityvac. But the guage is a must. You hand pump the vacuum unit and the guage should show the system holding the vacuum. If it does not, due to a broken diaphram, then the guage continues to drop to zero. If it is OK the guage should hold steady until you release the valve on the Mityvac. Sooooo! the bottom line is you want a guage.
I do not have a Mityvac brand. I got mine locally at a auto parts store. It works just as well as the Mityvac.

Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car