With the cap off and you are filling it, there won't be any air in the tank. Try filling with the nozzle about halfway out so the air can escape.The gas will push the air to the top. There has got to be something else wrong. If you are using your gauge as a reference as to how much gas in in the tank, then don't. Those old gauges are very innacurate. Try replacing the gauge with a new one, or at least replace the sending unit.