You may have a failing brake valve. The one I'm referring to is located at the rear axle or could be on the rear air tank. When you push the dash valve it activates the slave valve that directs air to the maxi brake chambers. These valves get old and gunk inside makes them stick. You just need to replace a bad one. You can locate this valve by following the air lines from the chambers up to the valve.
Eventually this valve will stick and you won't be able to move. Or you'll leave those long black streaks running to the shoulder.