Update for future reference: mobile mechanic came, rebuilt the rear caliper, new pads, bled the line. Two miles later: locked up rear brake AGAIN! Lines are fine, bled, completely new DOT4, new pads, pistons in fine shape, same outcome! Absolutely no problem bleeding the brakes so it’s not a problem with the flow to the caliper. We looked into the master cylinder and it was perfect. No blockage no residue. So the bike remains in Dallas and I’m in Houston right now for family reasons.
An interesting sidenote: the new pads, after they were seated in carefully, apparently only wore on the top half. Literally the bottom half of the pad was untouched. He checked the disc for unusual wear, and it was all within specs. Well with within actually. Visually it looks like a perfect disc, no waving or wobbling. Weird.
Long story longer, I’m going to order a new disc and a master cylinder rebuild kit, and we’ll see how it goes from there! It will be a few weeks before I’m back up in Dallas so there won’t be any updates for a while.