"After you hook up the Jeep, with the system completely aired up shut off the engine. Release the parking brake. The brakes should release, there should be a slight drop in the air pressure and then it should stop. The air pressure should hold where it is and there should be no sound of escaping air.
Then apply the service brakes and hold them. Again there will be a small drop in pressure as they are applied, but the drop should again stop almost immediately. Like before there should be no sound of air escaping."
What Jon has described here is critical and is worth repeating even if only 1 more member soaks it in.
This is a standard pre-trip inspection that should happen EVERY time you head out. It only takes 2 minutes. This is also the way we isolate a problem and helps pinpoint what.
Adding to this would be to air up and shut the bus off before you release the brakes, if there is a evident air leak at this point it would/could probably be in a suspension component. An air bag or a line to them or a ride height valve or norgren valve or possibly less common, a tank or tank fitting.
If all good, next, when the parking brake is released but before service brakes applied you shut the bus off and you encounter a leak that would be "emergency air" leaking, either a maxi chamber or an air line leading to them or possibly a valve.
If again all good then with the emergency brake off and the brake pedal applied and you shut the bus off and you hear a leak that would be "service air", possibly a brake diaphragm, (also incorporated into the maxi) or a line leading to that.
The maxi (brake chamber) has 2 air lines going to them. 1 is emergency air that airs up (by pushing the parking brake button) the side of the diaphragm opposite the large spring, just neutralizing (not overwhelming), the large parking brake spring thus releasing the parking brake.
For all you scuba divers think of it as when you release the parking brake your creating neutral buoyancy within the chamber by putting emergency air to the opposite side of the diaphram as the spring.
The other air line is service air and it is introduced (by stepping on the brake pedal) to the side of the diaphragm WITH the spring and that service air combined with the force of that spring DOES OVERWHELM that emergency air on the other side of the diaphragm thus engaging the brakes and with as little as or as much air as is introduced depending on how hard you apply pressure on the brake pedal.
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