Ran a line into the cabin so I could watch the vac gauge as I drove around. When I apply the brakes, the vacuum goes to zero. Started plugging through the FSM, what about the check valve? Any possibility that is the culprit?

Jay.

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On the road, currently:
'83 300D (German spec.) 168k mi. - - Wolfgang (tucked snugly away for the winter!)
'87 300D 375k mi. - finally went over to "The Dark Side"
'87 300TD 225k mi.
Cannondale 66cm CAAD5 w/Campy/Jamis Aurora
Hujsak Custom Fixie/Landshark Track Shark Custom/Ahearne Custom 29'er