Success!! My father talked to a plumber friend of his and got some kind of string and a white goop with teflon in it, and that did the trick.
We filled the system and saw no leaks. Then, for the first time in 17 years, started the engine and let it run for awhile. Still no leaks.
However, I'm off to the electrical forum for a question about smoking wires under the dash....
Thanks, everybody!!