At least 9 times out of 10 this condition is caused by an air leak in the system before the lift pump. If the leak were after the lift pump it would likely be a dripping fuel leak, but not always.
Do you get clear fuel to the engine mounted fuel filter (with the bleed screw open) when manually operating the pump? If not, start at the tank and check each connection and filter for proper seating. You may have to remove the tank pickup if there are connections in the tank above the fuel level. You may have to disconnect and reconnect each fitting. I would use diesel rated thread compound on threaded fittings. You could temporarily put some clear heavy wall tubing inline before the lift pump to check for air bubbles in the system. Good luck.