From my bad memory, factory pressure was 4.5 to no more then 5.5-6.0 lbs. But think it would at least idle at 2.5?
How i determine a bad line on a car that been sit'n a long time.
First check all hoses for dry rot/cracks. They can collapse internally when the pump puts a vacuum on them.
If that's not it.
Thinking safety 1st, disconnect line from tank at pump. Be sure to clamp line from tank so cant leak. I just use vise grips same as when replacing brake calipers. Then run a another line from pump to small gas can. See if it runs.
Had lines that had internal rust(plugging it up) along the frame rail. Outside rusty, but didn't look bad enough to replace.