While there may be some differences from year to year, the engine compressor goes through the air dryer then to the wet tank in the front of the coach, just behind the front axle. The 120 volt air compressor also feeds that same wet tank but not through the rear dryer on my coach. I have a separate dryer on the 120 volt pump. The wet tank feeds the front and rear brake tanks through hoses with check valves in them. The front brake tank is slightly behind the wet tank and somewhat above it on my coach. The rear brake tank is just ahead of the rear axle. If the check valves are working properly, the wet tank can lose air without the brake tanks losing air. There are also connections off the wet tank to feed all the air slide-out trays including the generator, any air slides in the bays, and the cover over the stairwell. Lines feed the air pocket doors (on those coaches that have them), the air-ride seat, the air-dump valve, the air horns, the air powered dash HVAC defrost vent directors (through a vacuum generator on mine), etc. Each of these has a separate line coming off of the air distribution lines and running to the various locations throughout the coach. Therein lie the problem. There are dozens of fittings with air lines running in multiple directions depending on where the air powered items are on your coach. Each switch has additional fittings then the connections where the air lines run to the air rams that move the slide, trays, doors, etc.
As an example, the air line that feeds my air-ride seat comes up through the floorboard in front of the driver then runs along the drivers side of the coach then across to the seat. Under the seat is a T fitting that supplies air to the switch that control the ride height of the base of the seat and to the switches that control air to each of the two bladders in the back of the seat. At each switch the air line comes in and one or more air lines go out along with an air discharge port. The potential air leak sources can multiple in a hurry.
Then you have the air system for the leveling system. That one is nice and complex. Richard is the master of the air leveling system but the very short version is air is feed from the wet tank into long rectangular leveling tanks that are mounted along the frame rails. These in turn feed the leveling control units (6 packs) which are controlled by the ride height valves, the level sensor and the master level control electronic unit. There is a 6 pack in the front of the coach for the front suspension and with a tag, two in the rear, one for the drive axle and one for the tag. Literally dozens of air lines and fittings involved there. Then you have the air fitting to the air supply in the engine compartment (typically) where you can air up tires or use an auxiliary air supply to fill the air leveling system. More chances for leaks there. I removed the fitting from the air supply in the engine compartment and put an inline shutoff valve behind the fitting so I could isolate that source of potential air leaks.
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