Let me add this, as Joel has been helping me with my 2011 bike that has been pressurizing the tank.
The pressure-vaccum valve should let air pass freely TO the tank, and slowly away. The check ball has microscopic pores that let air away from the tank at a very slow rate. Problem is, within 6500 miles and 1 winter's storage - my pores were starting to clog and were restricting flow away from the tank too much.
The ball can be removed. The cylinder splits into 2 pieces. This way, your tank still vents great, AND the fumes get taken care of by the CC.
Thanks go to Joel on this one, I'm just relaying knowledge to the collective.