Quote Originally Posted by firebird77clone View Post
Your vent may have a roll over valve which is sensing the rising feul level ( expansion) and closing your vent.
Installed correctly, the anti-rollover valve sits at or slightly above the top of tank, where it doesn't see liquid unless the system is in a rollover situation. A short brass nipple with a coupling elevates the valve. Too many people simply screw it into the tank, and then wonder why their system is building pressure. Like firebird says, the rollover check will float up, sealing the system if the fuel level is over the bottom of the valve.