The reservoir you refer to is for the self leveling suspension, plastic jar type thing with a cap and a dip stick. ASD and ADS use the same I dont know where your fluid went, one or both of the accumulators could have punctured. It is a semi bouncy ride if you go over bumbs, the problem is if you drive too long without the spheres functioning it ruins the shocks which are about $440 ea. There might be just enugh oil but does not show up on dip stick, get some from your MB dealer and see what happens. I recall there is a line on the stick that says "full when loaded". the system can be bled thru a port on the controller near the rear axle. ADS provides similar operation to the front shock and rear by electrially controlling the rate of fluid flow depending on the setting.