My dad restored my coupe back in the 60's. Rough fouling with the radiator a few days ago I cause a leak on the back side of the top inlet. So I took a wire brush to clean the area to re-solder. I discover there is some sort of a light yellow goo hardened. Some sort of a pourable substance that harden. I suppose it was put on way back to seal a crack.
Any idea what this could be? As I said, it had been water tight until I messed with it.
You will have to remove the yellow goo to solder. You could use black RTV to seal your leak. If you JB weld it then there will be more hard stuff to remove if you want to solder it later.