In the past, when the ABS light on the dash went on, it was that the brake master cylinder fluid level had dropped a hair below the 'fill to' line. Lately the light comes on and stays on and the master cylinder is filled just above the 'fill to' line. Does anyone know how the sensor works that determines if the master cylinder is low on fluid? Opened the cap on the master cylinder and there is a small circular item floating in the brake fluid. Is this the sensor? If so, how does it work?