Step 13: Attach the gauge
You have now finished your radiator pressure tester. It should look like the photo in step 1.
Attach the stopper to the neck of the radiator. See step 11. Attach a bicycle pump to the valve stem. See step 12. Your car's engine should not be running. Pump the system up until the gauge reads 15 pounds. Often you can hear a hiss where the system is leaking. You may see evidence of moisture appearing where the leak is located. You will also notice that the pressure reading on the gauge leaks down slowly after a few minutes.
Sometimes the pressure reading does not leak down, but you know your system has a leak. In those cases, chances are the leak is coming from your radiator cap. If the cap is a few years old, replace it and see if your loss of coolant has ceased.
I have used this tester several times and it was always a big help.