Originally Posted by autozen

...Hopefully you never let the fluid go so low that the pump ran dry, because the hydraulic fluid is all that lubes the pump.
(I've been pretty religious about topping it up and the passenger's side seems to be at the proper level. I have been wondering whether there is a high pressure return to the front of the car that may be leaking accounting for the fluid loss and listing to one side.)
The leak is probaly in the low pressure return hose from the pump to the reservoir.
(Is there a decent diagram available anywhere?)
If you think that one side is lower than the other (definitely noticable), you could have a failed hydraulic cyl (Pressure sphere?) on that side of the car.
If the car is sagging evenly, it could be the pump or the control valve on the rear of the car.(Not the case)
Real for sure diagnostics on this system requires a high pressure gauge with all the special fittings to test all the components. If the pump has been destroyed or the leveling valve has failed, you will need to get replacement parts from another 560SEL. Although the system functions exactly like the 123 wagon, the parts are totally different.

Peter