Where is the overflow tube attached to the radiator? If it's in the top tank, it won't matter what PSI cap you have- - - - -pressure will never build up at all. If the overflow attaches between the upper and lower flange of the radiator neck, then a spring-loaded cap will allow pressure to build up. Use a laser-pointer heat gun to check various points on the cylinder head, radiator tanks, hoses, etc. to get an idea how well the cooling system is functioning.
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!