It is not entirely a closed system. Over time, the seals do allow little bits of air to interface with the fluid, near the o rings and every time the top is opened, moisture from the air gradually enters the fluid. This moisture, over time, is what breaks down the fluid. Eventually the moisture laden fluid, when it comes in contact with the wall of the master and slave cylinders pit the wall that can abrade the seals & o rings further. Then they allow more moisture to pass into the brake fluid system.