Just finished my bike and after I ride it, the fuel tank hisses from the cap. Some fuel had splashed around inside the tank and spit out the cap. Got under the paint and screwed up my new paint job. Needless to say, I'm pretty pissed. Where is the vent on a MKII tank?
The vent is probably built into the cap. If it is hissing the vent system might be getting plugged up. Take the cap apart and inspect vent holes, passages, and gasket.
Also I recommend never filing tank more than 2 inches from top. I have seen a couple of my lacquer paint jobs ruined from sitting in the sun on the side stand with a topped off tank. Urethane clear is a lot more fuel resistant.