Brake Fluid Level Switch
WARNING: This page is about a different car, the 2019 Ford Fusion. However, it is still accessible from the selected car via links, so may be relevant.
The brake fluid level switch is a reed type switch mounted in the master cylinder reservoir. The brake fluid level switch is hardwired to the BCM through a signal circuit and 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 float drops, allowing the reed switch to close, pulling the reference voltage low. When the brake fluid level is high, the float lifts, opening the switch contacts, sending the reference voltage high on the signal circuit to the BCM.