Fuel leak

Does anyone know what causes fuel to run out of vent on tank sending unit . I got this 66 Barracuda with a new tank in it and I pulled it down to do some trunk repair and installed new gasket on sending unit and the new gasket leaked like crazy got that stopped now the fuel runs out of vent tube . I can put tube straight up don't help I put a bolt in it for now and parked it till I can find a fix. Hopefully someone has an answer . Thanks