I tested some radiator caps this summer, 2 used honda oem caps, one new honda cap and one used stant. the honda caps all let off pressure near the rated psi, the stant never let off pressure. I was using the stant cap on the car and it seemed to work fine, just not with the tester. I didnt trust it though so i replaced it with the new honda cap.
I had replaced my factory cap with a Stant as part of PM at 300,000 miles. The cap was defective and let the coolant evaporate out. I thought Stant was good stuff from back in the 70s when my friend's father used them on the cars he repaired in this driveway to earn some extra money. Never again.
I'd definitely replace the cap with the radiator. Might want to replace the upper and lower hoses and thermostat while the system is drained.