My tank also holds pressure. The vent is primarily for relieving vacuum, but it will release with enough pressure. However, it takes a lot of pressure before the valve activates, still its less than the tank cap obviously.

__________________
2009 E320 Bluetec 117,000
1995 E300D 306,000 Sold
1996 Ford Taurus LX 130,000 Sold
1985 300TD Still 225,000 Sold
2016 Ford Fusion 24,900