I spoke before of using pneumatic exhaust vents to vent gas caps. I like using the small oil bag caps from Bung King on the narrow tanks I produce. There are two things I change on them before I put them to use.
First thing I do is to chuck the filler bung in the lathe and machine a relief on the top surface so the cap o-ring sits in a groove instead of wanting to squeeze out when the cap is tightened.
Second is to chuck the cap and bung in the lathe and drill/tap so I can put a bronze media pneumatic exhaust valve into the bottom of the cap. This allows sufficient breathing in the tank while the bronze media keeps gas from splashing out. I've seen these exhaust valves in stainless too, I'm trying to source them out.