I am starting to build custom gas tanks and I have been looking at making the caps myself as the vented ones available are either too expensive or not worth the material used to make them. If anyone has any suggestions for vented gas cap designs that are leak proof I would love to hear it.
A low pressure check (one way) valve would allow air to enter the cap, but not allow fuel to leak. The carb should be able to hold back what little pressure builds in the tank from sloshing. I've seen such valves in 1/8" npt male. Some are even adjustable.