Make sure the lock nut is seated all the way. You need to tap it around with a punch and hammer to make it tight. Just a light tap, not a full swing! If you get it tight and it still leaks, take it apart and check the lip where the seal seats. Maybe there is a kink in the lip preventing it from sealing, or the old gasket is stuck in there. Also make sure that the back side of the sending unit does not have the old gasket stuck to it. If there was no gasket there when you started, the gas would have been pouring out of the tank, not just leaking.