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.
Do the job in a well ventilated space with NO pilot lights or other sources of spark. The fuel vapor is heavier than air and literally POURS out of the hole when you remove the sender. When I did it I was lying directly under it and could feel it flowing over my face. So . . . put out the cigarette first!