Most easy to flush the system with other than expensive "fantastic cooling system flush" products--simply use Cascade dishwasher powder or their liquid cleaner--For the liquid mix 2 capsulles of dishwasher cleaner to 1 gallon of water-dissolve thoroughly then filter the liquid thru a standard coffee filter-add to an empty cooling system. fill remaining volume with clear water(make sure to "burp" the system to get all air out. Start the engine and run till operating temp is obtained(thermostat open-run for at least 30 mins(best to take the rig for a drive)--When engine has cooled, drain the system(don't forget the block drain!)
Refill with clear water. Start and run up to op temp again--stop and let cool enough to drain. Refill with the coolant you choose(burp the system to get all air out) ---you should be good to go---If you don't use the Cascade liquid but use the powder, just make sure to get it all dissolved in hot water first--filter the same, and follow the same regimen----you will not believe the gunk this will take out of the cooling system!!