I had a tank vent problem on another bike I had and it would build pressure like that as well. There are three things you need to look for.
1. Kinked hose/vent
2. check valve installed backwards
3. The emisions canister can become saturated with gasoline creating a "wet rag over an open drain" effect not allowing any gasses from escaping. I would remove the evap canister (if you have one) completely to eliminate this issue all together.