The sensor is a type of pressure switch that shows and alerts a fault in the braking system.
[1] The fault could be lack of hydraulic fluid, low brake fluid in the reservoir, a broken hydraulic line or a bad master brake cylinder.
[2] The sensor has an input port to measure the hydraulic braking pressure and an electrical connector output for a wire connection for feeding the warning light.
Some brake failures do not trip the sensor into a fault mode.
[3][4][5][6] New accident avoidance technologies system also use brake fluid pressure sensors.