You have a leak. Although you think it only leaks when you top off, it will leak whenever the fuel is up against the sender flange and that happens even on a less than full tank when the boat is accelerated or when there is load towards the rear (like people). It could be as simple as a loose screw or a lillte more serious as in a bad gasket. The worst case is a bent sender flange or tank flange. Try tightening it fist but be careful, the screws can be broken faily easily. If that doesn't do it, you are going to have to remove the sender and look at the gasket/flange Thanks I try that