Mercedes-Benz W124 level control and pressure accumulators

The most likely cause is a pressure accumulator (level control sphere). Its diaphragm fails, oil fills the sphere and the suspension becomes hard. A pressure accumulator cannot be repaired, it is replaced. The factory has no replacement interval, and the spheres have lasted about 11–20 years.

The level control works with the engine running even if the pressure accumulators are broken, so the cause is usually elsewhere: a hydraulic leak (shock absorber rod seal, a rusted-through line or the hose from the reservoir to the pump), a detached or broken control rod, a blocked valve or a faulty pump. Check the oil level in the reservoir first.

Bleeding is done with the engine running. On top of the level control valve, at the control rod, there is a bleed screw, and the valve arm is moved to the up and down positions several times. Methods vary, so check the oil level at the end. The rear rises slowly, so give it time. After the repair, the height may need adjusting with the valve's adjusting screw. The level control is a pressurised hydraulic system, so if you are unsure, let a workshop do the job.

The system uses ZH-M oil or equivalent, not ATF. The reservoir cap has a dipstick, and filling is done to its upper mark: an overfilled reservoir spurts oil out from under the cap. The oil level should be monitored so that the pump does not suck in air.