Cat 416C

I have a Cat 416C that is having fuel problems. The operator stated that the engine died and would not restart. The upper fuel filter will not keep fuel in it, even when the engine is cranked the upper filter bleeds out, but the lower one is staying full. I replaced the fuel filters and the mechanical lift pump. I've checked all the fuel lines and seals also pressurized the fuel tank with air to see if fuel was getting to filter and lift pump. Any suggestions?

Cat 416C