If the gas cap in relatively new, it will have a plastic check valve in it. It has been my experience that this "check valve" is designed to prevent fumes from escaping the tank. I don't know if this was done on purpose, or by accident.
Regardless of that, an easy fix is to drill a simple 1/8" hole through the plastic piece. You don't need to drill into the center of this piece. There just needs to be a hole through it somewhere to allow the pressure to release
