My first question would be what are the symptoms and are you sure they're caused by it not venting?
Second question would be how you've checked the vent lines and made sure they weren't pinched.
I'm almost certain there's not valve anywhere in there, the only thing I can think is maybe there was a plug (or manufacturing defect/overspray/something else issue) blocking the vent from the factory.
Two easy tests I'd try: Try removing the hoses completely from the tank (just the vent and overflow ones) this will allow the tank to vent directly out of the nipple and completely remove a pinched hose from the equation. The second thing I'd try is blowing through the vent hose while its attached. This will show whether or not there's an obstruction in the vent line somewhere and at least help you move forward with remedies.
Love is the feeling you get when you like something as much as your motorcycle - Hunter S. Thompson
I just mı̸̸̸̸̸̸̸̸̸̸̸̸̸̸̸̸̨ade you wipe your screen.
-2009 Suzuki GSX-R 750 Race Bike
-2007 Honda 919
-1995 Nighthawk 750 (Tboned)
-1983 KZ 440 (Sold)