Low Brake Fluid
WARNING: This page is about a different car, the 2005 Saturn L300. However, it is still accessible from the selected car via links, so may be relevant.
Under low brake fluid conditions, the brake fluid switch closes. When closed, the brake fluid switch grounds circuit 333. The body control module (BCM) responds by sending a serial data message to the instrument panel cluster (IPC) to turn ON the brake system warning light.