The low brake fluid warning device (cars equipped) is contained in the reservoir of the master cylinder. If the brake fluid level falls about 0.70 in. (18mm) below the MAX line, a float closes an electrical circuit which causes the warning lamp on the dash panel to light.
Refilling the master cylinder will cause the warning lamp to go out. However, the entire brake system should be inspected for signs of leakage so the loss of brake fluid can be accounted for.