This is a copy of an earlier post I made on PS Fluid R&R. The conscious is that as much of the old fluid as possible is best removed given time and tool constraints and replacement with good quality "life blood" to avoid system degradation and resultant failure. As the heart of the system is a continuously running hydraulic pump and ensuring that clean fluid meeting required specs is cheap insurance for expensive parts. i use my 12V pump to drain engine oil and tranny fluid so it was a natural to make use of it in this similar draining job. Good Luck!

Power Steering Flush/
I just flushed and replaced filter on my SDL so I don't know how similar the Process would be but it worked well for me.
I used my home built 12v electric topside pump to remove the old fluid. Get the fluid warm first.

1. Disconnect the low pressure return line coming from the steering gear. I was able to disconnect mine by releasing a spring clip that hold the hose to a hard pipe fitting that empties into the PS pump sump. I disconnected it here because it was easy to plug the hard pipe quickly preventing fluid from flowing out and when you quickly elevate the return hose end you prevent fluid flowing out of that end.

2. Attached my suction pump inlet the return hose. A 3/8 OD polyethylene hose fit inside the return line perfectly.

3. Remove PS pump cover and start the suction pump. As fluid level in reservoir sinks stop pump when the filter is exposed and the fluid appears to be at the bottom level of the filter. You can look at the small space between the filter and the sump housing.

4. Remove the threaded stem holding the filter and the filter, replace with the new filter and reinstall the stem.

5. Top off the sump with new fluid. Start suction pump. the new fluid will be pulled through the pump, the pressure line, and the steering gear, out the return line by the suction pump. I pumped my old fluid into a clear 3 liter soda bottle. I kept an eye on the fluid level and added new fluid as the level decreased keep the level at 1/2 full. I had my daughter sit in the car and kept her turning the wheel from left to right and back as often and far as she could. When the fluid coming out of the return line was clean, stop the suction pump. System capacity is 1.2 Qt. I think I used about 2.5 Qt. during this flushing.

6. Replace the PS sump cover, remove pump from return line and quickly place the return line back on the hard pipe fitting that you have just quickly revved the plug from. Replace the spring clip securing the return hose to the hard pipe fitting.

7. Remove the PS sump cover and start her up. Have some new fluid handy and add enough to keep the fluid level at 1/2 full if needed. Have your helper turn the wheel from stop to stop a couple of times. Let the fluid get warm and then fill to the proper level. I filled to 1/2" below the top. replace the cover gasket, mine was hard and brittle. Put the cover on and your done.

8. I kept the cover on and sealed when disconnecting and connecting the return line in order to use the inherent vacuum to eliminate or reduce the flow of fluid out of the sump and return line. very little came out of either during the process. I placed my finger over the hard pipe end immediately and then quickly placed a rubber plug over the pipe end. I used a small rubber protective cap from some other part that I found and that fit snug you could make one easily by using a short piece of the proper size hose that has one end plugged. I used an outside plug because I figured it would be quicker and easier than inserting an inside plug into the pipe. After the change the new fluid was not pristine, a small amount of the old fluid was mixed into the new but was infinitely better than the old. I assume that getting all old fluid out could be accomplished with repeated procedures. Everything sounds and has been working great since. Some people blow off the filter stating that MB has eliminated them in newer cars but they filter only cost $2-3 so its cheap insurance for a PS pump or PS gear. My car may be different but the same procedure should work on most. Good Luck.

the rest of the discussion is here;
http://www.peachparts.com/shopforum/showthread.php?threadid=30904&highlight=power+steering+fluid