It is not a completely sealed system. It is only sealed from the reservoir down but not at the top of the reservoir. It has to breathe to allow for heat related expansion and contraction so the cap has a very small vent hole or slot. Moisture that is present in the atmosphere that is ever so slowely "breathing" in and out of the top of the reservoir gets absorbed into the brake fluid over time.It's supposed to be a sealed system except for the top of the reservoir, right? So where is this water coming from?