Fuel pressure problems 1994 f150?

i know this is my second post today, bear with me, this is actually my dad's truck that is messing up. i went over to his house tonight and got a first had look, first i turn the key to supply power, you can hear the fuel pumps working, being that its a two tank truck i switched between front and rear and tried to crank, nothing on either, i heard the relay myself, went to to the rail and we are getting fuel, just no pressure.. when you first turn the truck on and hook up the tester you have 40lbs or so of pressure then nothing, but the pumps are still running, there is no smell of fuel as to a leak or anything like that. we have replaced the fuel pressure regulator TWICE, once with one from advanced then one from napa just to see if maybe we had a bad one. so basically the problem is we are getting volume/fuel but no pressure.. spit me anything you got