you don't have to tamper with the fuel lines to draw air in. Corrosion in the pipes as they come out of the tank would do it. It's an easy thing to check. Fit a length of clear pipe between the lift pump and the filter, start the engine and look for bubbles in the pipe. Always start with the simple things.
try running it from a bottle of derv under bonnet-put lift pump feed into bottle and i/p return line-do all with clear pipe.if air clears up ,then leak is between tank and lift pump.
If air is still bad,suspect diaphram ruptured on lift pump-injection pump will start buzzing soon with that much air going through it-you have to have fuel only in the lines-no air