I have a fuel leak where the tube goes into the bottom of the gas tank. It is a aftermarket gas tank and I used a threaded brass nipple to go into it with a gas shut-off valve attached to it. I did use Permatex fuel resistant gasket sealer on the threads which are a pipe thread. It doesn't drip but it is just enough leakage to stink up the cab. I did empty the tank, disassemble, clean and reassemble, allowing about a week to cure. Any ideas on how to eliminate this issue?