besides the NORMAL flush with a hose running constant....
i have used , and will use , the plane WHITE gojo by the gobs back in the system,
sealed ,warmed, and OPEN it ALL back up and WATCH the crud flush out.
and then hose flush MORE rinse and repeat until your RESULTS are satisfactory to you.
then add 100% antifreeze (NOT THE 50% crap) for your region.