瞎哥好，见笑了

Another recommended simple way to detect leak is to apply soapy water to the nozzle of the valves.  I tried, doesn't seem to work.  

Anyway it's all based on my terrible experience with my AC units.  Eventually I learned that most of the AC guys aren't that smart.  When it comes to diagnostics, most of them don't have a clue.  Replacing parts is the best fix they can come up with.