[kitcarbp]

Rick, You are right, should not see siginificant pressure build-up in use, I am using an old school Mustang vented gas cap and filler neck on my set-up, the gas cap has a valve that lets air in and air out as I proved in my tests. (pics attached) I just dont have a feeling for what positive pressure the tank will reach in summer (if any) with the ATL flapper valve my cell fill plate has. Based on my test so far, I dont think gas vapor will be generated fast enough to have any positive pressure on tank (the bike pump method in my test puts air into the tank alot faster than gasoline vapor will be generated in use). I confirmed that the ATL flapper valve while designed to keep liquid from passing back up through the fill neck in the "unlikely event of a crash landing" , allows the gas vapor to bleed off through the ATL flapper valve's center mounting bolt, and then the vapor moves up the neck and out through the cap. Since this is my first custom tank/filler neck I'm figuring this out as I do my pressurized bench testing.
The goal here is to only have vapors escape through the gas cap and not have vapors build up in the trunk from any other leaks. I did learn that 1 psi is more than enough to test for leaks on this Polyethylene fuel tank. The tank bulges out on the top and bottom about 1 inch at 1 psi.

Regarding the leaks along the bulkhead connector threads, I figured out if I turn the bulkhead connector over so that the nut is installed from the underside or tank side, I will reduce one leak path through the threads. Rookie mistake on my part, common sense I guess. The stat-o-seal washers seem to to work great when used correctly.