you may be overthinking it - the vented cap is so as the gas level goes down there's no vacuum created which would make the fuel pump work overtime as the level decreased, until the vacuum created in the tank caused it to starve - the pressure buildup is prolly from temp expansion of the fuel - try opening it at the coldest part of the day [night] and see if it still does it - I'd think they'd not make it vent outwards to avoid letting the fuel vapors into the air, tho' my caps don't hold pressure like that, and you may have gotten a cap designed for a newer setup
if the fuel pump is good and there's no leaks in the system, it'll take a long time [if ever] for that line to the carb to lose enuff pressure that it doesn't spray when you undo it - it's the job of the needle n seat in the carb to hold that pressure back until the float says otherwise
Bill
