[xs-style]

Well brake lines will rust from the inside out. Brake fluid is ... I think hydroscopic is the term for it? Anyhow, it attracts water. Anytime the MC cap is opened, condensation is introduced. That combined with brake fluid change intervals that are often over looked can yield your results. Brake fluid also has a six month shelf life after being opened due to moisture exposure, but I digress.

If you ever get a chance watch it on YouTube where they do a cup test on brake fluid. They taker one cup of brake fluid and one cup of water in like take-out cups and flip them on top of each other. One leeches into the other and it's not long before u can't tell them apart, but you know the contamination is there.