T680 air system

I am having a new issue on my 2014 T680. My secondary air supply is building before my primary. I have no water in the system. My first thought is check valves. When draining my air tanks independently both gauges will drop to about 80 psi then only the tank I'm draining will drop to zero. In my mind it shows normal check valve operation. Is there any other fill control devices other than the check valves? Any input and guidance would be greatly appreciated.