My original radiator in the past developed pin hole cracks that leaked on the top tank. I took it to a radiator shop and he did patch them up while the radiator was in the truck. All they do is drain the tank enough so there's no coolant in the top, then apply some solder with a propane torch to the cracks. It lasted for a year or so, but pinhole leaks & cracks are really just messengers (like rotten planks on a ship) telling you that it's corroded from the inside out and soon you'll need a new radiator.