Like said, the master cylinder, humidity has gotten inside it. Probably will need to hone the body, and if corrosion damage is too deep then no seals on the planet will fix it. Commonly though you can get one hone/rebuild before the housing needs to be chucked. I have even reused the old seals if I can fold them all around to find zero cracks or corrosion dents and the rubber is still live. Reinstalled like that and hone only for no parts cost and the brake works for years. But then, I've done that type work so many times I can't count.
I'd go through caliper too like said, whatever happened to M/C has happened to caliper too, it just hasn't failed yet.