The fuel tank vent breather pipe has a two way valve attached, this is a small pipe running from the side of the tank and this is attached to the body.
Inside is a ball centralised by two springs, as the tank draws fuel it creates a vacuum and this moves the ball one way to admit air, if the tank becomes pressirised by heat and expanding fuel it opens the other way to let air out.

My synopsis is, either the valve is siezed in either of the open positions which is common, or the valve has previously been removed and the pipe is open.