tell shahnawaz ltd to take a hike, the PS flush is never part of any service literature of MB, the vital part of PS service in any MB of any era is the power steering filter change - the old models were really nice, there was round metal filter in the PS oil container on the pump, easy to change out and super cheap too.
the later models with the black plastic PS oil container have a filter but its built into that plastic box and cannot be removed, you can clean it out with brake cleaner or carby cleaner backwards and shake it to loosen debris - then let the container dry out and rinse with PSF. The usual procedure outside of Pakistan is to replace that box (its not very expensive either)
I have cleaned about a dozen or so and every one had a lot of "debris" in them depending on how old the car was - some had so much that it was preventing intake to the pump (had to buy new pump too)
If you install a new container box on the pump make sure to fill the container and leave the car alone for about 20 minutes so the filter can soak (this prevents hydraulic blowout) - then raise the car so the wheels are not on the ground - DO NOT start the engine and rotate the steering wheel lock to lock 10 times before starting the engine. Always make sure your PSF is full otherwise you will allow air into the hydraulic circuit which is a very long process in your car to remove.
