Okay, not sure if anyone's interested but I feel like I should finish off this thread. I split the block and,with some difficulty, removed the solid hydraulic line that runs from the pump down under the brake pedal and up over the trans to the sump. I blew it out, cleaned it, and got all the paint off the joint that I thought was leaking.
But there was no evidence of a crack or even a piinhole in the old brazing. There was also no rust. It was, to my eye, a beautiful (forty-year old) joint. I took the assembly to a friend who has an acetylene torch. He thought it looked perfect too and suggested it might be one of the nearby o-rings leaking, not the joint. I'll admit I was losing conviction but we heated it up and rebrazed it (with a brass rod) anyway. It wound up being a bit of a mess.
On my way home I stopped at the Kubota dealer and bought new O-rings. I got the assembly home, reinstalled the line with the new rings, topped up the hydraulic fluid, pushed manure around for an hour, hauled firewood, dragged a pasture. No leak. None. I now believe it was never the joint, but was that worn O-ring where the fluid managed to somehow migrate under the pipe.
The leak is fixed, at least for the moment.