Power Steering Is Tight, Chattering, What To Do?

I had the rack and pinion repalced, as well as the compressor unit, about 4 years ago, and have hardly drived it since, but the sterring got tight, and when I check the fluid level, I could see rusty water floating near the top, where the dipstick enters the unit. How did water get in there? Can I easily belld the system, and put in new fluid? I can't stand the idea of having an auto repair place do a simple procedure, and then charge me an arm and leg for it. Any ideas about the rusty water? I'm sure there's no way the water could have entered the power steering compressor at the filling cap, since I had that covered with a "cap".