The check valve on the booster vacuum line should not allow air to flow toward the booster, or vacuum to flow away from it. Got a new booster line; without connecting it to anything, I am able to generate 25 in Hg when I pump from the booster connection point.

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'Betsy the Getaway Car' : 1978 MB 240D, 582k
'The Mistress of the Sea' : 2001 Giant Boulder, 10.30k