You need you to disassemble the gas cap and remove the two small red diaphragms, assuming this is your only problem.
The diaphragm gets hard and losses it's flexibility and gets stuck, and doesn't allow proper venting.
IMHO this modification should be done regardless to prevent future problems whatsoever.
They are ment to vent the tank, but in the case of an accident not allow gas to leak out, but more often cause problems then they fix.
Here are some pictures to help you.
The plastic piece houses the two small red diaphragm pieces, this one being mine, they are already gone, and you can leave the plastic piece out all together, as I'm going to do now.
Some things to watch out for are, there are 5 small springs under the round collar, and a small round collar as well on the locking mechanism, don't lose any of them.