Air suspension plumbing questions

[Al Lewis]

I need some education on air suspension design on my 02 Monterey. I find two air tanks. On the dash air pressure gauge there are two needles, red and blue. They always read the same.
I seem to have an air leak that causes the rear suspension to settle to the stops in about 30 minutes. The front stays up much longer. Are there one way valves isolating various parts? 
I find four bleed valves in the engine bay but do not know where each goes.
By the way I just replaced both rear bags. It was not as bad as I expected but was tough on the body working on my back. I got parts from Michigan truck spring at $130 each.

[Mike Shumack]

Do you have "HWH Air leveling"?
If yes, your plumbing diagram would be similar to this:

Note the check valves "o>" at the tanks. They would keep air in the air bag system even if the air tanks drain to zero (assuming the check valves are not leaking).

[Al Lewis]

No I don't have air leveling.  But thanks for the diagram and the info on the check valves.