I got one for GSA once that had inop. brakes... Turns out that somebody had carefully removed the air activated stop light switch (the newer upgrade one that was installed on some trucks in place of the hydraulically activated switch) and every time that you hit the brake pedal, air just blew out the fitting... A plug fixed the brakes for the ten mile trip home, and a new switch from Saturn Surplus fixed both the air leak and the stop lights...
It could be a lot of things... Start at one end of the system (say, the pedal) and trace things through to the other (wheel cylinders, brake shoes, etc.) and see where the problem is...
Good luck,
