I don't know about the W140 specifically, but the system on older models consists of a hyraulic pump, a leveling valve, struts and accumulators. The pump provides pressurized fluid which is directed by the leveling valve to the struts. The leveling valve differentiates between components in the rear suspension to determine if the rear is sitting high or low. It adjusts pressure to the struts until the rear is sitting at the 'right' height. The accumulators are spheres with a diaphragm separating fluid from nitrogen. When the struts take a momentary compressive load such as going over a bump, the fluid compressed out of the struts goes into the accumulators. The nitrogen makes it progressively more difficult to accept more fluid in the accumulators so the car doesn't bottom out completely. The accumlators are the hydraulic equivalent of springs. There are coil springs on the rear suspension as well, but they won't support the car without the accumulators.
Older systems don't have the sophistication to level the car left to right. You never know with a W140. Older leveling systems have no reference to gravity so they won't try to level the car on a sloped driveway. Again, the W140 is not just any car.
When the suspension gets stiff, it's typically the accumulators that are failing or have failed. In most cases, the nitrogen leaks out leaving the accumulator full of fluid and unable to absorb the compression of the strut.
I don't know of any practical in-situ test of the accumulators other than a stiff ride. Any MB tech should recognize the symptoms immediately. MB recommends replacing the hydraulic system fluid periodically.