The valve at each wheel is tied together as one common system feeding it. You cannot control each wheel or axle independently with stock manifold and plumbing. All tires will equalize to same common pressure. The wheel valves pop open with a pulse of air when the pressure difference between the tire and feed is more than 20 psi. If the air is vented from the system at too high a rate, the tire valves slam shut. By rapidly dumping the air from the manifold, the system depressurizes the hub seals and isolates each tire for system shutdown. The system reduces tire pressure by letting air out of the manifold slowly so that the wheel valves don't close. The system monitors the air pressure at the manifold and because all axles and wheels are plumbed together, this represents the pressure at each tire - if the wheel valve is open. If a tire is flat or below 20 psi, the wheel valve will not open for that tire.
