2 tips:
1. use a NEW o-ring. The large rubber o-ring that goes around the unit. I have never been able to get a total seal with an old o-ring.
2. DO NOT use silicone on the o-ring to get a seal. it WILL disolve in the gas and at best clog your fuel filter.
good luck....if it doesn't seal this can be VERY frustrating....it's real cool to drive out of the as station and be dripping gas from the sender.