I think you are on the right steps, but I would replace that air catridge (concur with the rebuilt units being good to use) as soon as possible. Until it is replaced, you could just be recontaminating the lines and valves you are working on.
On the leveling system, also check the level sensors are set properly, replace or adjust as needed.
As you are aware, the air system is a safety item, and brakes could become a real problem for you and others with a bad air catridge in the mix
Best of luck, and let us know how things work out,
Also in an 04 Allure.