Originally Posted by rudaire
It would be nice to know that, I agree. But it's pretty hard to determine that from just a sound coming from the fuel cap. It isn't like a gush of air you can feel with your hand. It appears like my system doesn't have a provision to let air out of the tank, like if things heat up and expand.. But I refuse to believe there isn't a provision to let air into the tank to replace spent fuel...
If the sound occurs and the smell of gas in really strong immediately after, it's positive pressure. If it was suction, it wouldn't be so strong. With a pressurized tank, you could easily feel the air rushing out.