I would check the old rubber lines, if you have not replaced them. They are prone to collapse on the inside. Your tank does not need to be pressurized, but it DOES need to be vented. Try running the car with the gas cap on loosely and see if that makes a difference.
Also be sure all of your connections are tight so the pump isn't sucking air. It may help to blow out the hard line with some compressed air, unhook it on both ends and force air through from the engine compartment to the back.
