A agree with both of the above. I've repaired the steel line coming through the sending unit, as well as between the tank and the frame. I have also seen both the rubber hoses at each end of the steel lines cracked enough they didn't leak fuel, but could suck air. Look the fuel lines and the rubber in them over carefully. Probably wouldn't hurt to at least change the hoses.

Lets not forget that some carbs have a brass filter at the fuel inlet (especially if someone swapped a GM carb onto your truck), or an inline filter that could also be plugged.