Hope this helps. May NOT be specifically for your truck. http://www.suspensionspecialists.com/techinfo/Bepco_Air_Brake_Trbl_Chart.pdf
The solution is to find out where it's losing air. It it coming from the Air Springs, Brake Valves, etc.
If it only leaks when brakes are charged it's most likely a chamber... another common issue with leaks when engine is running is the fan hub