Current plan is to simply check (and probably reseat) every joint in the system. Is there a trick to finding an air leak?
As you suggest seems appropriate. The good news is the one you do last will be the one that was the problem. That is unless you forget one. You might look at the check valve on the pressure side of the pump first. Some pumps have this built in and if you have a City Fill connection there is a check valve some place. If you can pull the pump easily many can be serviced while some are totally self enclosed with no ability to service
. A little build up of rust or mineral from the water will do it.
When the check valve fails there is no water leaking from the system and you never see any water any place. The pressure just gets release back into the pump and the water goes back into the tank.
If you loose pressure in 20 minutes there has to be water someplace if it is leaking out a fitting. Any hose barb fittings that you disconnect regularly like for winterizing could be suspect too. You could leave toilet paper around to see if you can find the leaking water that way too.