x15 Coolant sys pressure test. Not losing pressure after 20 min

Did coolant pressure test. Found coolant leak from clamps being loose around hose from thermostat housing. Tighten them up and leak went away. I had it at 15 psi. It never came down from 15. What would cause it to not release pressure?

 

Also tested CAC and found a leak there. Not sure if that’s relevant to pressure not releasing

 

If the coolant system has no leaks why would it come down from 15psi?

 

Wasn’t thinking I guess. I had watched a video and remember them saying you pressurize it and wait 5 minutes to see if how much pressure you lose . So I assumed after 5 minutes you slowly start losing it.

 

Cooling system shouldn't loose pressure.
CAC on the other hand, we'll it shouldn't loose pressure but I wouldn't expect it to hold pressure forever. I mean it would be great if it did but some loss is acceptable

 

CAC has a pretty good leak. Can hear it hissing pretty and when spray soapy water it bubbles up good

 

What device did you use to pressure test the cooling system?