Some caps are marketed as "pressure/vacuum" and they vent only when the internal pressure in the tank goes above a certain level, or drops to a preset vacuum. Those don't work well in place of a cap with a simple vent hole. You can thank the tree huggers and their obsession with hydrocarbon vapors for that kind of nonsense!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!