Brake fluid is hygroscopic and pull water out of the air, and that causes the inside of the brake lines to rust slightly, plus brakes get very hot amplifying the amount of water pulled in as they cool. That is why you should change out your brake fluid every couple of years, depending on use. You probably have not ever noticed it but your vehicle is the same but with more fluid and a plastic reservoir, and it also needs changing when it starts to turn. That is just the nature of brake fluid.
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