Forrest, the red and green needles on your air gauge represent the air pressure in your primary and secondary air tanks. The red needle is your rear air brakes only. It will move only when the brakes are applied. The green needle is your secondary air tank which is your front brakes, air ride suspension, horn and other accessories, When driving, you will see the green needle go from 120psi down to 90psi and then back up to 120psi. As you drive the coach is constantly leveling itself so it uses air to do that. When the air pressure gets down to 90psi the air compressor on the engine comes on and builds pressure back up to 120psi and you hear the reciever/dryer cycle and purge contaminants from the compressor and dryer.
Safety system works this way, If the coach has an issue with the air compressor or you start losing air pressure for what ever the issue, you get a warning buzzer and light to alert you at about 70 to 80psi. If this happens, find a place to pull off the road safely. If you don't, when the air pressure gets down to 40psi it starts to apply the park brake. Now you will come to a stop on the road. Turn on your flashers, exit the coach and get away from it. Fred