Unencouraging update: I changed and tightened the lines on the primary filter, with factory brand lines, and checked with a mitivac to make sure air wasn't getting past the primary filter connections. I also changed the return lines with new factory line. I'm just getting froth in the line from the lift pump to the spin-on now, and I can see little bubbles coming into the primary filter, but from the lift pump side (i.e., against the fuel flow). One end of the hose is submerged, it is tight on both barbs of the filter (and the filter looks unobstructed). The fitting is as tight as I could reasonably get it on the intake to the lift pump, and I cleaned that fitting before putting the hose on it. I'm considering replacing the primer pump and see if that helps. I don't know if or how they break, but it sounds like an air pump when I'm pumping it ... granted that sound could be coming from somewhere else down there. Is there a way to test which part of the lift pump is leaking? I don't have the money to just buy parts I hope fix the problem.

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