p0455 error can't fill tank more than half way
 

my daughters boyfriend has a 01 or 02 gt and he has had a check engine light come up after replacing the fuel pump. his car comes up with a p0455 error code. the check engine light and check gas cap light stay on the dash. the car seems to run just fine other than the fact that if he puts more than half a tank of gas he will start loosing gas. it will start running out from the top of the tank. i lowered the tank down today and checked all his connections and didn't see anything suspicious.

anyone have an idea why it only leaks gas after a half of tank is put in? when it is below half tank there doesn't seem to be a single drop leaking