I wonder if sediment / rust has temporarily plugged the hole?
That reminds me of a lake in FL, Lake Jackson, that has sinkholes in the bottom: They plug up for a while, then one or more reopens and the lake drains. This now happens every few years. As it is one of the water supplies for Tallahassee, that’s a bit of a problem.
https://www.youtube.com/watch?v=3qQX8mLgst4
I have put stop leak in a couple of times. I think the stop leak has it’s limits on pressure. At least I know that a radiator is available even if I have to take it to a TIG welder to get it modified for the hose necks to be the correct angles. With a 45 year old tractor that’s not a popular USA model, there’s a lot that could go bad and there’s no getting parts for it.