Another possible answer is some gas tanks had a vent tube that vented the tank. It could be plugged. I worked in body shops when I was younger and have seen many tanks vented this way but do not remember what brands they were. good luck

The pressure build up is most likely due to having the improper gas cap installed. The original caps were vented, to allow the tank to breathe. With the advent of overall vehicle emissions control the factories went to non vented caps and some form of a vapor recovery system. The non vented caps would allow air to enter the tank but not allow fumes to vent to the atmosphere. Check to see if you have the proper vented cap. My guess is you don't.