Beige - I'm a little confused.. the gear box and the P/S reservoir share their fluid? I'm just unsure how the system works (I'm new to working on cars, but hot damn, i love every second of it). I was gonna drain through the return hose on the P/S Pump. Is there one method that is superior to the other?
Also - Yesterday I picked up some NAPA branded P/S fluid to put into my car. I'm changing over from the ATF. Should that fluid be OK for these old Benz's?
MBeige's way is the correct way to do it and it is how the Mercedes-Benz dealership would do it. The way I told you to do it is a general practice common to non-MB shops and many DIY-ers. It's the way my father taught me to do it. I have used this method to convert every MB I have ever owned and I have never had a problem as a result. The way I told you to do it will not hurt anything so long as you make 100% sure to get all of the old ATF out. Having said that, if you want to do it the way the MB FSM tells you to do it, I would follow MBeige's instructions. MBeige is a very well respected MB guy who has helped me out a ton, so I would never contradict his mechanical advice. If you are new at working on cars, it is a good idea to get into the habit of following the FSM. Mercedes-Benz is the oldest car manufacturer in the world and they know what's best for their cars.
NAPA P/S fluid will be fine as long as it is fresh from a sealed container (like any other fluid, if the container has been open for extended periods it will become contaminated with dust and moisture). Generally speaking with P/S and Brake fluid, the most up-to-date formula will be fine in older cars. With coolant, oil, additives, and ATF I ALWAYS follow MB guidelines, but Brake and P/S fluid are pretty close to universal. Modern manufacturing and refining processes exceed the standards set in the 1970's and 1980's.
And yes, the P/S pump circulates fluid through the steering gear box. It is a hydraulic pump that utilizes hydraulic gear suspension principles to create a "power assist" in turning the worm gears inside the gear box. It is the same principle as quicksand or an air hockey table. In quicksand, the natural flow of water in the depths of the sand make the sand "fluffy" and that's why you sink. Your body movement adds to this effect and that's why people who panic and flail around sink faster. The same with air hockey, the cushion of less dense moving air helps the puck to float. In both examples of the same principle, one joule is multiplied by the physics of hydraulics. This is the least confusing way I can describe this principle (I am an engineer, but I have never been a good teacher).