I think you're onto something! After I read this I remembered that many times when I'm going to remove the fuel cap I hear a lot of air leaving the tank (I think it's coming out because I can smell petrol). Today I went for a drive and after I parked the car for a while, removed the fuel cap and heard the rushing air, started the car and it started immediately, but didn't tried to start before removing the cap... Apparently the tank isn't "breathing", the fuel cap has some holes in the middle but when I tried to blow trough them I couldn't at all. Do you know the function of those holes is to allow air in and out?
Thanks