The rear should not be sagging unless you have weak springs or the system is not working. There is a container under the hood for MB hydraulic fluid, with a dip stick, it also contains a filter to keep the crud out. The fluid travels from an engine mounted pump to a valve mounted on the rear carrier, with an arm attched to the rear toshion bar. If the car sags the fluid is pumped to the spheres (the spheres are containers filled with nitrogen and a rubber diaphram, the pressurized oil is on the other side of the nitrogen). The spheres are attached to the hydraulic struts (shock look alike) so the pressure of the oil and dampening of the accumulator controls the level. The stuts usually dont fail, unless they leak, the accumulators do go, as the diaphram ruptures it will be spongey, when there is no more gas left it becomes very stiff, and then you could cause damage.
When the torshion bar twists it lets in oil, releases oil, or in the netral position bypasses it back to the reservoir. it should be easy to check disconnect the control lever, the car should rise or fall, some control valves have a index for the neutral position.
This applies to 123 T, 126, and others. On 140 body cars the pump is mounted in tandem to the power steering, and on some the pump also provides some anti lock in the differential. On ADS vehicles there is an electric valve to control fluid flow to the struts, and solenoids attached to front shucks to do the same. On some 129s they use struts on all four wheels