gas ventalation system/gas leak

about a year and a half ago I changed the gas tank on my hatch because it has a rust hole in it. Then a while after I started getting leaks from the filler tube when it was full, I tried tightning it down and it seemed to solve the problem. Then the problem happened again a few weeks ago, so I checked the filler tube (where I could see it was coming from) and tightned it again, and it was fine for a while till today when I knowticed a puddle (small, but still) under my car (again). However last night my car was full and didn't leak at all, but it leaked after driving around for 20 min. I am thinking it is presure related, I took the gas cap today and some gas spited out (along with a loud Pisssss). I was wondering if anybody had any ideas on how to solve this?? I have read that the ventalation system for the gas tank sends the excess vapours to the carcoal cannister up front (along with the ones from the intake manifold) do you think it could be that its cloged and there is too much pressure?? or maybe its the valve that controls the flow to the canister?? or am I missing something completly?? anybody got any ideas???

thanks

Put a match to it and see what happens....

Put a match to it and see what happens....

how about we try it with your car first?? anybody have any USEFUL ideas or sugestions??

have you tried replacing your gas cap? If they dont vent right then it will cause either excessive pressure or a vacum inside your tank.

I did that today (new gas cap, was $5 at local canadian tire, it says its emisions certified, is vented and designed for the car) I will try that and post the results after driving for a bit, thanks TheSilentChamber....

UPDATE, the new gas cap did nothing, after inspecting around the tank again, its obvious that the leak is coming from where the tank connects to the 2 way valve wich opens if the presure gets to high, so I am thinking the valve isn't working right creating too high presure, causing a leak.....I am going to try replacing the valve and see what happens....