My neighbors problem cub had a leak in the radiator that would only show up when the radiator was warm. It would set for days cold with no problem, but as soon as it warmed up it would leak (non pressure cap). Of course the local radiator shop didn't have any way to pressure test a cub radiator unless we toook the lower tank in also. I suspected the leak was in the area of this old repair, but with the fan sucking air through the radiator it was hard to tell for sure.
So I decided to try a little experiment. I drained the radiator, and set the pressure regulator on my compressor to about 5 pounds.
I then put an open fitting on the hose and inserted it into the top of the radiator and stuffed a wet rag around it to make a loose seal.
I turnd the air on and sprayed it with a mixture of 20% dish soap and water in a hand spray bottle.
Maybe the leak is not where I thought it was.
OOPS, forgot to put the drain plug back in.
Replaced it and look at the bubbles.
With this set up it can be soldered and retested without refilling.