The ABC system on Mercedes Benz vehicles is a computer controlled hydraulic system, the system adds / removes fluid from the struts, the objective being to keep the chassis level at all times. This is a great system when fully functional.   

A common failure point of this system is the valve blocks. This manifest itself as one or more corners of the vehicle sagging or lowering when the vehicle is parked after a few hours or days (longer term sagging isn’t necessary cause for concern). Or one or more corner raising shortly after vehicle shut off. 

The valve block fails to make a good seal allowing fluid to escape the strut and return to the reservoir leading to the corner sagging. This failure to seal is more often than not caused by the O-rings deteriorating within the valve block solenoids, most likely the locking valve.

There are two valve blocks on each vehicle. One controls the front struts the other the rear. Each valve block has two locking valves and two control valves. The locking valve locks the level of the strut once the vehicle is turned off.

If any residue is stuck in the solenoid the plunger will not seal correctly and strut fluid will be lost resulting in that corner of the car sagging. Such residue is normally caused by the original O-rings deteriorating. Alongside the O-rings are backup rings to maintain the shape of the O-rings under pressure. Some valve block solenoids are fitted with nitrile back up rings, like the nitrile O-rings these deteriorate and can clog the plunger.