Coolant Pressure Test Questions

[Spanky1]

2015 3500 6.7
There' is no radiator cap on this year.
1. Is there a check valve in the coolant resrvoir tank overflow line that keeps it pressurized?
I attempted to pressure test via the tank cap as I have seen done on numerous videos. However, it does not tank any pressure and the gauge doesn't move. I overfilled the tank so I know it's not split.
2. Any ideas why I can't pressure? The tester does work as I tested the cap itself and it releases at 21 lbs

 

[crash68]

Is there a check valve in the coolant resrvoir tank overflow line that keeps it pressurized?

That's not a reservoir tank, it's a degas bottle because it's an active part of the cooling system.
Is the overpressure drain being blocked off when your attempting to pressurize the system.