I myself applied pressure to the tank in the manner I sugested. It is not some theory. I did not clamp the return line. I know nothing about the system being a closed loop. Air in the fuel line is released when the banjo bolt on the fuel filter is cracked open. I tried this method when I had trouble getting fuel to the filter after a fuel tank screen cleaning, hand pump replacement, and filter change. I suggested this method when Azitizz said that he had had similar troubles, and that no amount of pumping seemed to prime his engine. Try it, as long as you don't go overboard with the air pressure, what harm have you done. If it's going to work you should know very soon after you crack open the banjo bolt. Good Luck, Dave