Brake Fluid Level Switch
WARNING: This page is about a different car, the 2017 Lincoln Navigator and 2017 Ford Expedition. 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 separate signal and return circuits. The brake fluid level switch is grounded to a body ground through a separate circuit. The BCM provides a battery reference voltage to the brake fluid level switch. When the brake fluid level is low, the float drops closing the switch, pulling the reference voltage low. When the brake fluid level is high, the float lifts, releasing the switch contacts and removing the ground to the BCM.