[Richard]

I can’t remember what I just had for breakfast, so I made this label that resides on the overhead console along with the label of my coach height.  Sorry about the pic quality. 

   

When the switch is in the center position, the tag bag receives the same pressure as the drive air bags; they are pneumatically tied together. Earlier coaches had a second sixpack in the rear which allowed you to adjust the pressure in the tag bags separate from the drive bags. 

So how do the switch do what it do? The key is two three way electric solenoid valves in the line between the drive bag air tank and the tag bag air tank. Keep in mind this setup is mirrored for right and left sides of the coach. 

One of the three way valves is plumbed so that in NC the pathway is open from the drive bags to the tag bags, the alternate port is plumbed into the air supply. So that when the switch activates this valve, and the valve goes to OPEN, the pathway to the drive bags is now closed and the supply pressure starts to increase the pressure in the tag bag.

The second three way valve is also plumbed so that in the NC position, the pathway is open from the drive bags to the tag bags. When the switch is activated to switch the valve, the open port is to atmosphere, so that the pathway to the drive bags is now closed, and the air in the tag bags exhausts to atmosphere. 

One caveat is that if the switch is left in position to let air out of the tag bags, the normal system pressure of 125 is NOT enough to support the weight of the coach on the drive bags. Left this way, the coach will eventually sit on the drive tires. This is not theory, I custom grooved some tires in such a manner.