Brake Fluid Level Switch
WARNING: This page is about a different car, the 2018 Ford Edge. However, it is still accessible from the selected car via links, so may be relevant.
The brake fluid level switch is mounted in the master cylinder reservoir and is hardwired to the BCM through a signal circuit. The brake fluid level switch is grounded through a separate body ground circuit. The BCM provides a reference voltage to the brake fluid level switch. When the brake fluid level is low the switch closes, pulling the reference voltage low. When the brake fluid level is high, the switch opens, sending the reference voltage high on the signal circuit to the BCM.