Jim's got the picture. There is a flexible metal piece at the top of the cap that seals the top of the filler neck as the cap is tightened fully. It would also be possible to drill the top of the radiator cap for a long sheet metal screw that would retract the spring loaded center of the cap slightly to prevent pressure buildup. A factory 0-PSI cap is a better choice, though.
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!