when that happened to me ,it was summer and hot..gas expanded (before I got a vented cap) and blew the seal/gasket at the sending unit...

I`d check and see if you can find where it leaks (do NOT put a match there to light with!), if its the neck or maybe you have a gasket thats bad to the sending unit (if so the gas would come down over the top)...its very hard to fix a leaky tank as you cannot weld on it...if neck is broken , I`d buy a new tank...if its the gasket at the sending unit..its cheap, and all you have to do is drop the tank....run it almost empty first ,that tank gets heavy when you loosen up the straps holding it!