I can chime in with a bit of scientific background here. I worked in a test lab last summer and did a lot of work with corrosion. For rate tests we needed to mix up a solution that would eat away the corroded material but not the base metal. So if you ever want to clean rust off of your steel, mix up some Hydrochloric acid with stannous choloride and antimony trioxide- 100ml, 5g, and 5g. That will eat iron oxide but not iron/steel.

Guess what we used for copper? 1:1 HCL and water. It will consume copper oxides but not copper. So mix up HCL (aka muriatic) into water and use there. 1:1 is probably a bit strong for radiator cleaning- when I dropped a copper test coupon into a dish of 1:1 HCL:water, it became clean bright copper within about 2-3 seconds. You probably are more patient than that, right? You'll do fine with a weaker solution.