Less than a cup of acid in 3 gallons of water isn't a terribly strong mix. If you let it sit in an aluminum radiator overnight, it might do some damage, but 30 minutes or so of circulating with the coolant isn't going to do much damage, especially since the next step is a flush with baking soda to neutralize the acid residue, followed by a clear water rinse. Like with a lot of other pretty aggressive procedures, a little common sense goes a long way. That's why commercial "flush" products have been defanged- - - -far too many people were too stupid to use the strong stuff wisely, and manufacturers are scared to death of government regulators and legal beagles.
Jerry
