Coolant pressure test question (on procedure) when there is no radiator cap.

[2004mountaineers]

In just 5 days of short-commute driving, I noticed that the coolant level started from 75% of the way from the bottom line to the top line (closer to the top line) to instead all the way down to the bottom line. Both measurements were done on a cold engine.
I pressurized it to 15psi as shown in the included pictures and did not see any coolant coming out. There was already a wet spot as annotated in the picture below. The wet-spot did not seem to grow by pressurizing the reservoir. And, it held 15psi fine. Normally I would pressurize the radiator, NOT the reservoir, but this vehicle does not have a radiator cap. That is where my primary question lies -- am I even doing the pressure test right by pressurizing the reservoir ?

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2004 Mercury Mountaineer, 4cyl, v6, 'awd'

 

[fordysenior]

I come from 'old school' pressure testing where the radiator has a cap on it. I have my own tester, old school. I had to go to O'Reillys and borrow a tool to connect like you are connected. So, as far as I know, yes, you are doing it right. But as I remember, 18 pounds was the common pressure to use -- also, pressure test the cap to make sure it's holding.

 

[BrooklynBay]

Make sure that the system is completely full with no air gaps when you apply pressure.

 

[fordysenior]

Another thought came to mind -- the "leak" might be in your heater core. I once figured that out when I turned on the defroster and the inside of my windshield turned to a layer of moisture.