I've used it on car radiators and such. A small PC radiator won't need anything more serious than this. Just pressure test the radiator and find the leak. The Quiksteel is a two part epoxy, just cut off the amount you need, knead it to a uniform color and push onto the leak. Sets in ~an hour.
Best/cheapest fix. A car shop won't fix it, they dont fix radiators, they just buy new ones. A local Jr. college with a welding program would fix it. Here at our college I've had the students weld/braze (sp?) up some parts for me. They enjoy the practice and dont charge, and I get it fixed for free.
But the QuikSteel will work. Found at Advance Auto Parts/Autozone for <$5.
Edit: This is better than JB Weld. Jb is more liquid and runny. This quiksteel is a paste, much more workable, and cures faster.