How is your pump outlet secured to your supply nipple ? Perhaps the connecting hose has blown off allowing fuel to pump up and out back to tank sump or possibly a ruptured hose doing the same thing. Do you have a pulse damper on the pressure side of the pump. What type of pressure regulator are you using. Mechanical or vacuum. If a vacuum is used apply 20" vac and see if your pressure doesn't rise slightly. If mechanical, try capping the return the run the priming cycle and observe your pressure. I believe, by your description, you'll find a leak at the pump in the tank.