[ohcbird]

It sounds like you have an air leak somewhere along your lines. try to see if you lines alone will hold a vacuum. Disconnect them at the rear sump (obviously, cap those), then see if you can pull a vacuum on your system. The interior of the lines/hoses behave differently under a vacuum than under pressure.

My customer's 66 'Stang had a short line (1yr old) that was collapsing inside & doing just about the same thing, but at random intervals.