[url=http://www.overclockers.com/beginners-guide-water-cooling/#assembly]Assembly and Testing: Preparation[/url] said:
"Radiator preparation is one oft-missed item. Boil sink water and let it cool 5 min. Pour into the radiator, filling it up and let it sit 10 minutes. Drain half of the water or so and shake it till your arms hurt, shaking 3-4 minutes, like a crazy person. Drain into a clear container. Do the “radiator dance” again and again till the water coming out is clear and there is no gunk once the water settles. Then do it two more times. Finally, fill the rad with distilled (or deionized) water and do the rad dance one last time. NOW and only now is your radiator 90% clean. No worries, the last 10% will come out in the next year or two when you redo your loop for maintenance.
Inspect your parts. Open the pump and look for gunk / packing material. Run sink water through the blocks, pump and hose. It would probably be a good idea to run distilled/deionized through these as well. Drain as well as you can, but don’t go crazy on draining every last drop. Inspect bottom of block. Don’t forget to remove the plastic cover! Inspect screws and holes, checking to ensure they go together well first. DON’T put a screw through the radiator; I’ve seen this done more times than you’d think."