Brake fluid is Hygroscopic (not Hydro..) just to start meaning it will pull moisture from anywhere including the air.
Moisture may also be created from heating and cooling. The brake fluid gets hot enough to turn contained water into steam which is why you lose brakes and, in the case of a Wing, you lose the clutch.
Moisture is also pulled right through the rubber lines.
If brake fluid looks bad (discolored), it has been bad for a long time.
I have done brake jobs the right way, and several people said they almost put themselves through the windshield. This is because they got use to the brakes getting worse a little at a time.