...and I would suggest that you detach the upper radiator hose at the inlet: this will prevent backflushing any debris into the radiator or reservoir tank .

By the time you are backflushing there shouldn't be much of the old coolant mix left in the system, if you have adequately removed it from the engine and heater by draining & refilling, getting up to normal operating temp and re-draining the system. Maybe twice?

I then run the backflush into a large tub until I see no evidence of color nor particulate coming out. A pint of diluted coolant further diluted by 16 quarts of fresh water is not very toxic nor dangerous. Put it on the flowers.

You may be amazed at the amount of gunk and/or grit (scale) that can come out of the system, depending often upon the level of previous cooling system maintenance.

Cheers!

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'91 420 SEL @ 199K, '92 SVX @ 181K, '93 SC400 @ 86K, '93 Kaw ZX-11 @ 30K, '87 F250 @ 181K , 2001 Valkyrie Interstate @ 6K, Y2K Honda NightHawk 250 with 1.5K, '88 420SEL I.@ 179K & the 2nd latest, an '88 420SEL II.@ 210K runnin' parts car, '85 F150 300/NP435