Air can only come in via suction, so the leak allowing air IN is between the tank and pump. If you had a leak between the pump and carb there would be fluid coming OUT of the fitting as that's the high pressure side of the pump.
You could have any number of issues on the suction side...pin hole in pickup tube, dry rot rubber line between tank and frame (10% ethanol can cause this), loose fitting, stuck check valve in the pump etc.
Did you check the pressure and flow yet?
Just for the record, I didn't tell you to hit the pump with a hammer...lol!
Mike B
