Update: I had the brake lever pistons lock up twice on my long ride yesterday. (Brakes were working normally at the start of the ride.) The first time was about 4 miles after the start of the ride, after a short downhill where the brakes were lightly used to control my speed. I was in a pack and as we came up to a stop sign, I pulled on the brakes to slow with the pack, and both brakes levers were locked up. I veered out of the pack, while pulling harder on the levers, and I felt a slight "pop" and they were working again.
The second time was at the top of a 6 mile climb where I hadn't used the brakes in quite some time, I pulled on the brakes at the top of the climb, and only the right lever was stuck. (The one I purged with new mineral oil ) The Left/front brake worked normally.
It is becoming clear to me that there it is something in the brake lever hydraulic/piston side of things that becomes stuck. As soon as I can allocate the time, I will have to take one apart and see if I can find what is causing this problem. On the flip side. The ride had over 12K of climbing/descending, and on all of the long high speed descents, the brakes worked really well without any problem.