Some of you are partially right. Some of you are completely wrong. This is an open system. If it were not, you would never be able to adjust the system pressure. There is a reservoir which is initially charged with 175 psig of N2. From there, the compressor introduces up to 220 psig of compressed atmospheric gas as needed. Over time, all of that N2 will be replaced by atmospheric gas, although there is a desiccant integrated to aid in drying of the compressed air.
If all system pressure is lost, it must be recharged with N2 manually. Aside from the fact that N2 is free from moisture, this is a safety feature as well as a diagnostic aid. The computer will not allow the compressor to run if that N2 precharge is not there, because it knows there is a leak in the system and the compressor would just run and eventually burn up trying to replenish the reservoir.
There is a manifold or "valve block" between the reservoir and the components which houses solenoids and isolates each component (downstream of the reservoir and manifold) so that if one component leaks, it will not deplete the entire system of pressure, but will let you know there is a problem by throwing a code. There are pressure sensors (as well as level/height sensors) on each component and the computer continually monitors and compares all of them for mismatches, much like speed sensors or any other integrated system sensors.
Here is a small quote from the manual:
"OPERATION
The air suspension system is an open system. The air suspension reservoir stores a specified volume of pressurized air for use when the air suspension system needs to raise the vehicle height. If a single component is damaged, the reservoir will hold any pressure left in the component. A scan tool is used to evacuate the component. After repair or replacement of the component, a scan tool can be used to refill the component from the reservoir.
The compressor cannot refill the reservoir and complete system if all pressure is exhausted, as in a compressor assembly or valve block replacement. The system will need to be filled at the reservoir with nitrogen. For complete filling of the air suspension system after a repair See: Suspension Control ( Automatic - Electronic )\Service and Repair\Rear Suspension - Complete System Fill. "
