[G.M.]

You didn't say where the filter is located? If after the flex hose the hose could have a small hole in it. These holes don't leak fuel but do suck air. If the gas is low in the tank air can be drawn down through the gas when the pick up tube has dirt over the end. On pumps with glass filters attached the bail nut and cup on the bottom gets so hot vapor bubbles can be seen coming up from the bottom in hot weather. Some gaskets don't seal the glass top and that's where most bubbles come from. G.M.