2000 S500 car is very low in front and back. It will raise some days when I crank it, but will go back low in a day or two?

yes. apparently there is a slow leak. With a floorjack and a squirter bottle full of soapy water, you could spray it down and watch for where's it leakin' from-

If you begin to drive before it is all the way raised, the air spring bladders will rub on other suspension parts and make any leak you already have worse, But it could be hoses, plastic air lines about 3/6 ID that run from compressor to bladders and self~ leveling height sensors, The squirt bottle is an excellent idea, bubbles will appear. But because it is both front and back, it's probably the air lines, small they are...but unfortunately not exposed all the way back. And where they are hidden is most likely place to rub and make a hole or slit. Start by squirting the compressor line fittings, Does NOT need much, don't soak everything, it's an electrical device

see David, I DID NOT know that! air, couldn't possibly be electrical especially when faced up to road and salt spray...typical built-in failures.

pneumatic fittings are on the devil's own pricing schedule, cannot begin to imagine the cost incurred from this non-duckable suspension problem-

this one blew me away....so much for Union Workers puttin a sandwich wrapper inside the door on a Friday production. http://www.youtube.com/watch?v=uuDeEArJ8NQ&playnext=1&list=PL0CA338ABD1E646FC&feature=results_main

I'm all for paying a decent living wage to anybody for an honest day's work, but $55 an hour to watch an assembly line, no skill needed? Unions...have a purpose, but the Indians are telling the Chief how to hunt buffalo

Could you imagine the level of responsibility...down to a millimeter at speeds that a motor can live with, programming---which I'm sure will snap the golden handcuffs on you!

one guy welding in an oxygen suit, and the other guy hawkin' it up as if on drugs...ya see that? typical British.

Not sure if you're still dealing with this issue. Its either 1) Your compressor is bad. The compressor is located in the front right section behind the bumper. If you kick the bumper sometimes it will jump your compressor. If you listen with your ear to the bumper and you hear a "piff piff" sound, then its likely your compressor. You can pick these up used for about 400.00 at the junk yard. 2) Your suspension system is bad. If this is the case its a lot more work and the cost can range up to 6K.

When the front air springs leak out on those cars its the caps at the top of the strut. If you look under the hood at the strut towers find where the small white plastic air lines attach to the brass fittings. Originally there is hard black gel substance encapsulating the fitting and the cap. Look for cracks there and spray soapy water on it. MB has a repair kit for those (may even be covered by "goodwill warranty"). Otherwise the leak is most likely the valve block which is located just above the air compressor. Spray soapy water on that. If you deal with road salt chances are that valve block will be quite corroded.