Pictured is a commercial radiator pressure tester. They are relatively expensive for the DIY home mechanic. But, there are times when it would be very handy to have a radiator pressure tester for finding leaks leading to coolant loss and possible engine overheating out on the highway. (The photo is from Google images.)
Step 1: My homemade tester
When we had one youngster in college, another in high school, and my wife was working we had four cars in the driveway for a while. We had another child out in her own apartment, but she always brought her car to dad when it needed to be fixed. It was not uncommon for one of these cars to have a mysterious coolant system leak. I made this pressure tester so I could locate and fix such leaks quickly.