A buddy of mine is doing the cummins swap in a 2000 f250. We have everything done, engine and trans all mounted, wired up new guages, completely ready to drive except for one problem. We cant get the fuel to stay primed. We can put the fuel line in a bucket and she runs great.when it goes to pull fuel from the tank(and yes there is fuel in the tank) it keeps grabbing air. We have been checking for air leaks. cant find anything.we've bled the line over and over and still cant hold the prime. anybody with any help/information/pointers, please let me know. thanks.