Thanks for the tip, JB.
After decades without any trouble, I had
six "bursts" this year—all in a tiny, cramped space, of course. They're all soldered now, at some risk to burning the place down.
Apparently, the concrete hot water heater base had settled (due to the rains and wet soil), pulling down the piping and setting up a series of low spots.
I'll go look for those splices for another season, but what's considered "pricey"?