My gage has two needles, a red one and a white one. The way this was explained to me, was that the white one was the "normal" air and the red one is the emergency brakes? I don't know, the previous owner did it. It originally had two pressure gages up front, he changed one to this dual gage, and hooked the other one up to the brakes so you could see how much brake pressure you were using. I'm going to press it up and pay closer attention today.
Is it 3psi per minute with the park brake set, or brake pedal held down? I will have to get my wife to hold the brakes while I crawl all around underneath with my hearing aids turned up.
I wish I knew more about air brakes. I already talked to the local truck/rv place here; they are going to do a safety inspection and brake tutorial with me as soon as I can get it there.
Try this. It is the DOT test!
I'd also note that it's important to do these checks often.