Maybe they mean the fluid in the reservoir?

 

Probably because the filter screen isn't replaceable. That's my guess at least.

 

Power steering maintenance is the first thing I did (after CPS replacement) to my rover. Admittedly all I did was a thorough fluid change with Valvoline MaxLife fluid but it helped a lot. I removed as much of the old fluid as possible with a pump and filled it with fresh fluid. I then took it for a local drive and did a bunch of figure eights in a parking lot (I suppose this could be done parked in the driveway as well). It's important that you turn the steering wheel back and forth many times so the fluid in the power assist cylinder has a chance to purge out, which it only does when the wheel is actively being rotated. I repeated this process 2 more times, sucking out the old fluid and replacing it with new fluid every time. Yes, I did go through several quarts of fluid but I'm satisfied with the results.