What vehicle are you testing, and is the radiator original to it? 1950's cars and trucks would run 7# pressure systems, while 1940's and very early 50's vehicles sometimes used a "zero-pound" system. The heater core is often the limiting factor on an older vehicle that has had the radiator swapped for a higher pressure item. Modern radiators and heaters can handle 15 PSI or more. Over pressuring an older system can spring leaks or even burst seams. Every pound of pressure on a cooling system raises the boiling point 3 degrees F. so a 7 PSI system boils at 233 degrees at sea level, lower at higher altitudes. A 15 pound cap raises the boiling point to 247 or so.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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