I am repairing a truck and cant figure out why the fuel filter is half full. installed new lift pump. Read as much as i could find in this forum and a air leak was the cause. I ran a clear line from the tank to lift pump and there were very small bubbles running through the line. Can this cause my problem also the injection pump goes dry. i.e. fuel line from filter to inj. pump removed no fuel comes out of either end. I checked vacuum lowest was -4.5 in third gear and full throttle up hill. Is the spec of -4 at idle or full no load or full load? any help would be appreciated. p.s. the air bubbles that other guys have found coming from the tank, can you describe how much air was actually entering? thank you.

 

You have an air leak in the suction line. That will cause all the symptoms you are seeing. Have you pressurized the fuel tank (only a few psi) to see if you can find the leak?