have yo verified visually that it is truly full. if the vent which is normally turned up in the tank so that it is at the top of the tank is instead turned down then fuel would come out of the vent considerably earlier then the tank filling up. if this is not the case visually inspect the lines for a micro nick. I had a small nick in one of my fuel lines with a ys pressurized system. could not find the leak easily but raw fuel on the aircraft pointed to a problem. only found it when it was running and tank pressurized. came out as a very fine mist which did not effect engine running since it was so small and hard to barely see but it adds up over a few minutes