I'm almost embarrassed to close out this thread by telling what happened, but I will....
Took my car to a repair shop, and it turns out the flare nut wasn't tightened enough! I thought I had twice tried to tighten it down, but I guess I didn't try hard enough
. Either that, or the mechanic's SnapOn tools were a little better than the Robogrip I was using to squeeze the flare nut and tighten it.
Oh well, live and learn.