Thanks, Gordon.
So I took the rear brake lines out this a.m. to look for trouble. Couldn't see any nicks or chips or cracks. I took the T and barrel junctions to Pep Boys and left there with a spare barrel—no new T—and the new barrel looked a little rougher than the one I had.
So I hosed all the threads down with Brake Kleen and reassembled, using a new bit of tubing for the cross piece between the barrel and the T going to the right side (the original of that looked like it mayyybe had a crack...? Couldn't tell, so I fetched a new one from my box-o-brakelines.
I assembled the whole thing on the bench. Tight. Tighter.
Then I installed it back in the car, trying to the the main line back in as tightly as the the rest.