The simple answer in lieu of the much longer one you probably deserve is that the green needle reports the front tank pressure and the red one the rear.
The air dryer puts hopefully dry air into the "wet" side of the front tank, where any remaining moisture supposedly condenses out as the warmed air cools, but if your dryer is working properly the air should be free of water before it gets anywhere near the front tank. It then passes to the "dry" side. A pressure protection valve there sends air to accessories such as step cover, horn, etc., as long as the pressure exceeds 65 lbs or so. This assures there is at least some pressure for rear braking regardless of accessories. The dry side also supplies a height control valve for two air springs.
The rear tank is fed from the front one's dry side via a one way valve, so pressure loss in the front will not diminish the rear tank's. The rear tank supplies two height control valves, for the front air bags I think, at least that's the way I understand it - others here may like to elucidate and correct that notion. I also am led to believe the rear tank supplies air to the front service brakes, and there is probably a 65psi protection valve involved for that line in that tank like in the first.
Others here may also wish to speculate as to why the rear tank may have bled down more.
Joel
