Are you using the new locking ring? If so, use your old one, or bend the tabs on the ring to a more extreme angle than they're at. They are too shallow so the lock ring doesn't put enough pressure on the gasket. I had the same problem.
The fact that newer lock rings are stamped out of thinner metal or not stamped correctly is a well documented problem. We chased a dripping sending unit for two weeks. Tried different gaskets, doubling up gaskets, sealer, etc Put the old lock ring on the new sender, no leaks! :mumum:
