Most air brake systems are pretty generic. There are two basic systems, the cam and lever type, and the "wedge" brake system. Cam & lever is much better, IMHO. Can you post a pic of the part that's leaking?
The basic idea behind air brakes is a pilot valve operated by the foot pedal that switches main line pressure on and off the diaphragms in the air chambers. There will be relay valves close to the wheels, controlled by the foot valve air signal. It's sort of like a low-current relay controlling a high-current device like a starter solenoid, except with air pressure, a small amount of air volume can be applied, not a wide open or off situation like a solenoid does. Sometimes the relay valves develop leaks that can show up a long way from the malfunctioning valve because the pressure backs up through a control line and finds an escape route through the exhaust port of the foot valve or a dump valve for the air chambers at the wheels. Have fun tracking the leak to its source!
Jerry
