Sounds like the builder neglected to put in an anti-rollover vent valve, which has a small plastic ball check valve that floats to the top to block off the vent if the tank is over filled, or falls to block in the event of a rollover. It also vents as you fill the tank, keeping it from burping back at you through the filler valve. You need to drop the tank and do it right -

antirollover.jpg