I did this same thing a couple of years ago but when I looked at the lines at the rack there was no way in hell I was going to disturb that mess of rust and scale. I did flush the system but did it a little differently.

I took the feed hose to the reservoir and re-routed it into a large waste jug.
The suction hose got routed into a large jug with fresh fluid.
It was a little messy but worked well enough and saved the headache of messing with the hard lines.
BB