yea a couple of problems, and some suggestions. one its hard to contaminate a power steering system, its not normal, one thing id check color of fluid, if its normal color, i doubt youd want to, need to flush. one thing if you did flush, an older system, a good chance you washout an old, and/or cracking seal(s). it already sounds like you *may have a problem. i have several cars with p/s, or one make or another, and the vws with p/s i have. i cannt remember the last time ive had to add fluid. over so long, yes once in a while is not a cause,. but it sounds like you, before putting new fluid in, you may want to trace down for leak(s). most likely seal(s). p/s seals can be a b*tch. the drain is all the way under on the opposite side of steering rack. car has to go up on stands. a couple of things,, first, when you drain system, and put new fluid in, unhook p/s pump belt, you have to turn p/s pump(pulley) a few good times, to prime, otherwise youll run it dry, and eeeh,or screech, it wont make it. another when you get enough fluid in it, as i dont have enough direct experience, have someone else IN car, and start car, be ready to fill reservoir, if fluid starts to drop,quickly, you may have to right then have wifey shut her off, and then add fluid, do again, until fluid level stabilizes, have to also prime, fill rack. after that, turn steering wheel a few times, to work out air-bubbles.
use good fluid, i prefer a syn fluid, but thats me(strong).