Originally Posted By: Phil in MI
Other thing which is puzzling, is the tank has to be full for the engine to prime, otherwise its squishing air in the prime ball.

Hope its something easy.
It's more telling than puzzling. Look at your fuel tank, does the return line go back in about 1/4 of the way down? If so, I'd be looking at any check valves in the fuel system. It sounds like it might be sucking air back through the return somehow or the priming pump is leaking.