A place for more technical discussions. Please make sure you post in the correct section on the site, this way it keeps the site tidy AND ensures you get a more relevant answer.
Actually thinking about it, that vent valve in the tank is likely designed to stop fuel coming out in a roll-over, when gravity is in operation , rather than if it is over filled when the right way up.
If it suddenly breaks, go back to the last thing that you did before it broke and start looking there