Thank you for this info! I have begun the process to order new cartridges for all 5 single handle valves in my house.What brand of shower valves do you have? If it is a common valve and the cartridge is reasonably priced, then you could buy three and replace them all at this point. Or if it is a brand that has a test cap that can be put in in place of the valve, then you could get a test cap(not sure they sell them) and pull the valve and replace with the test cap and re-test.
Look in the description, it comes with a "Heat-resistant pressure test cap allows for air or water testing before valve cartridge is installed"