I suggest the solution (sorry, pun not intended) is multiple flushes. I'm a Chemist, and there's a common practice in Chem Labs when one is cleaning apparatus. This applies to how to rinse off whatever was in / on the apparatus, especially after washing it with some cleaning additive. Empty the system as well as you can, then refill with fresh clean water. Run the system for a while to rinse all that fesh water through it, then shut down. Repeat the drain / fresh water fill / circulate / drain process seven times. In almost all cases this will remove everything and leave you with a clean system you can fill with real fresh water (or coolant mixture if you're using additives). If you're really keen, do it a couple more times than seven, but usually not necessary. I do not suggest high pressure anything for this - you could damage your equipment.