With the engine idling, turn the steering wheel from lock to lock 2 or 3 times and set the wheels to go straight ahead.
Turn off the engine.
The reservoir is mounted on the power steering pump. Wipe the reservoir, unscrew the nut and remove the cap from the reservoir.
Check the fluid level, which should be between the MIN and MAX marks located on the plastic sleeve, which is attached to the stud for fastening the tank cap.
(Article obtained from this portal: Mercedesman.ru)
If necessary, add liquid to the tank to the required level. Install the cap and secure it with the nut.