why does my gas tank keep leaking?

Did you place the larg O-Ring seal onto the pump module before installing the pump module into the tank.. .. Or .. did you lay the large O-Ring seal into the tank and then install the pump module?.. Did you clean//remove ALL debris from seal area before install?... Did you put a little lubricant on the seal to aid in installation and sealing?... Are you SURE it's not the fuel tank pressure sensor grommet that's the culprit to start with?.. It's right beside the pump wire connector on top of the module and easily dislodged when installing tank!... One more question... Does it leak ONLY when completely full of fuel?

IF it is in fact leaking from the seal you just replaced , then you only have 2 choices--defective/incorrect part , or improper installation.

Not blaming anybody but, My problem 2000 Express original fuel pump Assembly E3971M (And many more) has the Vent tube coming from the Fuel Pump to? or Plug blew off and of course is leaking fuel out the top of the tank, now. Plugged that up, leak Mystery solved AND tank pressure P0452 ODB2 trouble code Mystery problem solved as well. Happy Days!