
By saying " have air " do you mean you have more than 85 PSI showing on both gauges ?
While driving down the highway you should mantain 100 to 120 psi.
After repeat brake applications you may drop to below 100, if the pressure drops below 65 psi you get the low air alarm , about 50 psi your spring ( parking ) brakes come on and you come to a stop until you get 100 psi again.
If you have over 100 psi and the alarm goes off you have a sensor problem.