Removed the master cylinder assy today and brought it home. Not the easiest to take apart, but when I did there wasn’t a clear failure to point to. There is a check valve in the design and I still suspect that was hung open. The primary cup seal looked fine. There was no deep scoring of the inside cylinder wall. A little wear was evident, but not likely an issue. There was a very slight indentation on the small o-ring that seals the check valve, but even that didn’t look like it was worthy of mention. Maybe a small piece of dirt got in somewhere that wasn’t obvious. I put it all back together and figure I’ll get some more Dot 4 and see what happens in a small bench test at some point. Overall a nicely designed unit so kinda odd that it failed.