If the fluid level in the brake system reservoir drops to the maximum permissible level, then a warning lamp lights up on the instrument panel. The signal to it comes from the brake fluid level sensor located in the reservoir cap on the master cylinder. Wipe the lid and reservoir with a clean cloth, removing all dust and road gravel. Rotate the cover 90 degrees and lift it up. Under the cap, you will see a rod with a round float on it. At the base of the rod is a switch, and in the float is a magnet. As soon as the liquid level raises the float up the rod, the switch contact opens. Lowering, the liquid level brings the float closer to the magnet, and the contact closes, turning on the lamp.
Since the sensor and float are connected to the cap, the only way to replace them is to replace the cap. Disconnect the wires from the connector (connectors) on the cover and replace it.