I run a bottle of white vinegar through the cooling system before refilling it. The acetic acid in vinegar cleans/removes rust & deposits from the block & radiator without harming any cooling system components. After the cooling system is clean I refill it & add 1/4 cup of pepper for good measure. Pepper is the best cooling system sealer there is. It seals radiators, heater cores, small head gasket leaks, etc.. without harming anything.
My S-10 radiator developed a pinhole leak 2 yrs ago, I dumped 1/4 cup of pepper in it & it hasn't leaked a drop since. It also works great on heater cores. My 94 YJ's heater core had a pretty good leak & lost about 1/2 gallon of coolant a week. I dumped 1/2 cup of pepper in & it never leaked again, that was 4yrs ago lol!!!
Sometime you gotta raid the grocery store for good car care products... :lol:
Hope this helps,