Yeah I have that problem with WW as well.... I a two year old system was heavly gunked over... My solution was to drain, refilled with 60% water / 40% "DRAINO". After a while I noticed the gunk slowing coming off. When you sqeeze the tubing the area which changes shape the gunk flows right off. I was going to run it for a week and let the gunk flow off by itself but i got impatient so i sqweezed all the tubing and cleaned it right up. BTW I am using clearflex. Now I flushed it out twice with tap water and in a few min i am going to flush it out with distilled water then refill with distilled water and a small (teaspoon) amount of "DRAINO". The only problem that it has bleach in it and I hear bleach will eat through plastics and rubber......? I am not too worried about loosing heat transfer or capacity considering I have a 120W TEC and duel temperature monitors. (coldplate, water temperature)
It would of been much easier just to replace the small amout of tubing but I wanted to remove the gunk in the blocks, pump, and rad to prevent as much as i can of contamination of the new water.
LMK what you think...