well I would assume the tank is supposed to have a vent somewhere or maybe the cap is supposed to be vented? reason I ask is every time I take my cap off lots of pressure escapes. is this normal or do I have a problem?
I have had the problem with the cap not venting. When you open the tank you hear a rush of air going IN to the tank. They say you can soak the cap in SeaFoam however did not work for me. Need new cap. I was bad, I sliced my gasket for a quick fix. BTW if you go on a 100 mile ride with the cap in current condition, the bike could die on side of road. (until you loosen the cap to let in some air)
pressure is normal, vacuum is not. If you take the cap off when it's cool and alot of air sucks in, then you need to clean the cap, most often you can take it off and rap it a few times on concrete and then blow air in all of the holes.
On the inside center of the gas cap there is a hole that vents through two outlets just outside the rubber seal. Blow through the center hole to check if it is stopped up. If no air flow you can usually soak in any solvent and clean with compressed air.