Thanks for the reply. From the looks of your answer I'm not quite sure if I've explained my question correctly (perhaps a moratorium on posts at 3 AM is in order?).

I was trying to test the booster line itself, to see if it would hold a vacuum. So I broke the connection at the booster and connected my pump to the line there. (This is where I think I might have been unclear; did my first post sound like I was trying to test the booster?) What I noticed when I started pumping is that I was able to draw air through the valve in the line, away from the vacuum pump and the door lock circuit.

Seems like the valve should be trying to maintain vacuum in the booster instead of letting vacuum out, back toward the pump. I'm trying to figure out if this is how it should work before putting down some money on a replacement line.

Sorry for the length. Apparently I like to see myself type.

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