Just thought I would pass along a little information that I learned. The radiator cap for a 2007 Prius is 16401-20353 rated at 88kPa or 12.76 psi. This surprised me because I am used to seeing pressures somewhere in the neighborhood of 15psi. Why the lower pressure, I can't say. There are parts outlets that will sell you (this will fit) a cap that is rated at 108kPa or 15.7psi. In fact it is very common on other Toyota engines. Here is the real kicker, this is the instruction detail in the factory service manual. "Using the radiator cap tester, increase the pressure inside the radiator to 177 kPa (1.8kgf/cm2, 25.6 psi)" Make of it what you will.
It says that in the 2006 RM (for one) that is floating around. The purpose of that instruction is to find leaks so the pressure has to be far higher than it would be in normal operation. The testing scenario does not relate to the normal operation.
You do know you have a pressure cap because when the water's under pressure the boiling point can be higher without actually boiling that's how the whole pressure cap thing came into play I do imagine so 3 lb probably won't make a humongous difference in a radiator system in a car or in most cars so 12.7 versus 15 lb and using a test of 25 or 26 lb as you can well see is not going to be really problematic The 12 lb cap is probably giving you ample boiling headroom without any extra pressure The 15 lb will give you some more boiling headroom with 2-point whatever more pounds in the system probably not going to affect very much at all but the boiling protection you get from the other 2 lb I guess to somebody could mean something so the manufacturer I'm sure tries to pick the lowest pressure with the reasonable amount of boiling protection without putting undue pressure on the system causing it to need excessive maintenance and the repair. People used to play with radiator cap pressures all the time and now they don't
