silverbenz22

i have noticed lately that just the rear has been sitting low but not low enough where i could drive it or the light coming on just low. When i hit the rasied button it goes up a little bit but the car doesnt seem to get level and the front works perfectly with all the different settings. I was wondering if you think it is the rear valve that is going bad, bad seals, and if it is how much would that cost and could i do it myself? It is a 2002 S430 with airmatic and it seems to only drop when it sits.