I recently added a cork seal to the cap of my fuel container and now it might be sealing a little TOO well. We recently had a big temperature swing and I just noticed that the tank had enough pressure built up to give it a 6 months pregnant look. I've never had this problem before as my tanks didn't have such a perfect seal. Do you leave the vent cracked slightly to relieve pressure on plastic containers? This is an old one, and there is no way to relieve pressure except manually.