It could just mean somebody turned the cap upside down and left it that way
long enough for brake fluid to dribble into the cap. This would not affect
the electronics, so your brake warning lamp problem is not due to that.
[you don't say what make model, mileage, age, etc....]
question: is this the original master cylinder or a replacement? many
of the after market cylinders have very low quality float switches and
the light will commonly come on without reason.
1. replace with oem, or
2. unplug the leads and make sure you regularly do a visual check on the
fluid level instead.
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