Loosen the fuel filler cap just to establish the tank is not vacuum locked. If the tank vent is obstructed they can do these type of things. You could also consider putting a line pressure gauge on to see if that lift pump is history after that. I would gravity feed it from a container to make sure the test is absolute. It does sound like you know about air leaks already. The foaming you see in the pre filter could be air introduced by a leaky primer pump or just agitated diesel fuel possibly.