Only did a bit of work today. Disassembled the other two master cylinders to check for corrosion before ordering replacement parts from Matco. This one had substantial corrosion around the top.

It was enough that after removing the retaining ring, it didn't want to come apart. I used some scraps of wood to create a little toggle mechanism to lever it out.

Yep, pretty corroded. The cap O-ring was sheared during disassembly. I'm pretty sure I have a replacement O-ring. I ordered replacements for the three non-anodized parts of each master cylinder. $120.