Old and brittle, when I tried to open the radiator drain, the 'nut' just broke off. Anyone have this happen and if so, what is the best way to remove the rest of the 'valve'?

 

Pull the radiator and see if you can remove the piece inside

Having had this happen on a few vehicles, just replacing the radiator is best, the fact it was stuck enough to not unscrew usually means the lower radiator is clogged up and can not be flushed clean