If its a copper/brass style radiator,you can solder it with 50/50 lead/tin solder,or the same stuff you use on house plumbing copper tubing--you cant really braze it,because the radiator will melt long before the brazing rod will--and you cant weld copper or brass that I know of,only solder it...
If your radiator is aluminum,I've used a product called "Rub-On" you can get at welding shops--it looks like strips of aluminum,they have been advertising a similar product on a infomercial on TV lately,the TV ad gets 45 bucks for 2 lbs worth of rods,I got mine for much less at the welding place.It works great on radiators,heater cores,aluminum window frames,canoes,gutters,anything aluminum,zinc,or galvanized steel..they show a bellhousing being repaired with it on the TV infomercial--the stuff is very strong,and it only requires a propane torch,and no flux--I've seen it advertised in the Northern Tool catolog also...I'd use that on an aluminum radiator over epoxy or stop leak--the best stop leak I've used is powdered Aluma-Seal,comes in a clear plastic tube.....