Thor 1211 stated that any air movement when opening the cap is bad. Not so. It is very appropriate for some pressure to be present. Note that I did not state they will operate forever with no problems.They are "not so smart" if the spring mechanism doesn't work for any reason. Mainly corrosion is the culprit I always spray a little WD 40 into the tiny hole on the spring mechanism, and then push in and out many times to lubricate the inside of the cap. Had many overflows of gas on a hot day because of a faulty spring mechanism. Lesson learned.
Bill
Gasoline will develop pressure when "stirred up" and your findings are completely normal. Vacuum, however, is something you should never encounter.The gas cap could be the problem. After sitting for 24 hours, there is zero pressure when opening the cap. When running just for a few miles there is noticeable pressure when opening
RootesRooter; yes the dizzy came with a new cap
Bill