The power steering reservoir on the W211 & W219 platform have a filter built in. This filter is not serviceable, meaning that you need to replace the reservoir if you also want to replace the filter.
I took the liberty to replace the reservoir/filter and also flush the complete system and put fresh new synthetic oil in.
I used a vacuumpump to get most of the old red mineral fluid out. Then replaced the reservoir and used the “lock-to-lock” steering motion to pump all the old fluid out. Temporarily cap off the return line on the new reservoir and extend the return line to any suitable container for the old fluid.
I used about 2 liters to flush the system and 1 liter to fill the system. Febi Bilstein CHF 11-S Nr. 06161 is the correct fluid for this system.
Steering goes noticeably lighter after this service!