Coolant leak

Somehow I became responsible for several 912 ULS equipped aircraft at our airport, not complaining, I'm enjoying working on these precision engines.  My most common problem is coolant leaks.  Like I learned in auto shop we used a pressure tester to place the system under pressure and find the leaks.  What size is the expansion tank neck and how much pressure (PSI/Bar) should be applied?  Any suggestions on test equipment any of you have used successfully?  

I have successfully pressurized the coolant system through the small overflow port in the side of the expansion tank neck with the pressure cap in place.

There is near Zero Resistance to flow INTO the tank, and 18 psi hold off to flow OUT of the tank.

I set an air-line and regulator up for ~20-25 psi to pressurize the system.

Any major leaks will be readily apparent by spraying a little soapy water on the connections.

Once I believe all the leaks are found I pressurize it one more time and then turn off the Air.

If removing the cap after an overnight rest results in a satisfying pressure release (whoosh!), I declare the system Leak Free!