Re: gas tank pressure question
Most caps are vented, thus eliminating the need for an extra vent. Are you making your own cap or buying a non vented one? If not, you should not need a vent. The check valve sounds like a bad idea to me, but I'm not a tank builder. If it is a spring loaded check valve, it requires a certain negative pressure before it will open. I don't know, but it seems like it could starve your engine for gas, maybe, before it opens. It would require a very sensitive spring.
Caution! I give opinions as if they were worth something. Use at your own risk.