I am confused about the ventilation system in the new plastic tanks. I have a new tank. Never had fuel in it. I took off the cap for the first time today and the pressure that had built up sounded like a space shuttle launch. Startled the shell out of me. Here is what I think I know regarding new tank regs. No more venting. The cap allows some sort of valve. Help me please as I am used to using the old metal vented tanks.