Try pulling the valve stem core out of the fill line and give it a drive for being an unloaded experience. Even with the air pressure at 0 there is air trapped in the bag. With the valve core out of the stem there is no trapped air as the open valve will work as an exhaust for the trapped air to escape. At least it works that way in my mind.
Another route maybe to replace the rear shocks with a shorter shock. By dropping the rear end 2" to get a level stance you have already reduced the amount of bottom damper by 2" of the factory shocks. Again, just thinking out loud.