sl 600 ADS bouncy bouncy
The sl600 does not have air suspension. It has a variable stiffness hydro-pnuematic system. There are nitrogen charged 'accumulators' for each wheel that provide the 'give'. Thus far it's just like the rear shock system on 560 model 126's. (These accumulators cost about $120 each and are the green spherical double fist sized steel balls in the trunk and in the front fender wells. When those get low in nitrogen, the car rides very hard.
The added wrinkle is the ADS - Active Dampening System, which adjusts the shock stiffness (by controlling th eflow of oil from the shock to/from the accumulators to best control the bumps and keep the tires on the road. There are acceleration sensors front and rear to sense body movements and the electronics adjust each shock to keep the car smooth. There is a switch on the console which allows you to set them to a 'sportier' (stiffer) setting.
It sounds like your rear shocks do not have any oil in them. Have you checked the hydraulic oil reservoir?
If the ADS valve is leaking badly, if may have to be replaced. You might first see if replacement seals are available. It could be the rear valve has failed completly open, which would allow the rear end to 'bouncy bouncy'