I found this in custom pc a while back and just thought ide post is here, its quite a good, simple guide to clean out your loop.
http://www.custompc.co.uk/howtos/601551/clean-a-water-cooling-loop.html#
Open the link and click launch this guide, it will open in a seperate window.
http://www.custompc.co.uk/howtos/601551/clean-a-water-cooling-loop.html#
Open the link and click launch this guide, it will open in a seperate window.