Fuel pressure problem

[dubie]

As i drive the truck more and more, we keep working out the bugs that comes after a fresh build. One we found a couple days ago had to do with the fuel pressure. We filled the tank, which is the original behind the seat one, went for a cruise and parked at a buddies house. When his neighbour came out to see my truck, I opened the door and it smelled like fresh gas in the cab. SO I checked the tank and the fitting was leaking. The strange thing is, it only does it after the truck has been running and has been parked and shut off. I checked and the fitting was a bit loose so I grabbed a couple wrenches and tightened it all up. Then I took it for a ride last night, come home, park it in the shop and go into the house for 10 minutes. I come back out and it was slowly dripping past the flare and the fitting. I assume this is the fuel making it's way back to the tank from the line. Which leads me to belive that this was the reason why the truck had a tough time to fire after sitting for a couple days, all the fuel was going back to the tank.
So my question is, is there a back flow valve I can install to prevent this? Or should I look towards my tank and why it doesn't seem to vent properly? This is a new one for me.