[timwilky]

My dad used to own a radiator repairers, so I have plenty of experience as a kid of repairing rads. There is only one sure method. A re-core.

Otherwise, you need to remove the top/bottom tanks and then soak the core in an acid bath to clean it.

Then a blob of solder at either end of the holed tube. Reassemble core/tanks.

To test, blank hose fittings with a rubber bung, a washer either side and a nut/bolt through. As you squeeze the bung, it expands to seal the hole. fit an air line with about 3-5 psi and drop it into a water tank and looks for leak bubbles. If OK, refit. Careful with the pressure you do not want to rupture the core and make sure those bungs are tight. I have seen them fly 30ft smashing a window on their way.

Sounds like hard work, it is. that is why it costs for a repair. Especially with modern plastic tanks that need to be bonded. Old fashioned soldered was far easier.