Circuit/System Description
WARNING: This page is about a different car, the 2008 Cadillac CTS. However, it is still accessible from the selected car via links, so may be relevant.
The brake fluid level switch monitors the level of brake fluid in the master cylinder. When the instrument panel cluster (IPC) detects the brake fluid level switch input is grounded for 15 seconds, it turns ON the brake warning indicator and sends a GMLAN serial data message to the EBCM that the brake fluid level is low.