If the level of brake fluid in the main brake cylinder is insufficient, the contacts of the sensor installed in the reservoir cover are closed, and the indicator lamp on the instrument panel lights up. To check the health of the sensor, it is enough to press on the axis protruding from the tank cover. If the lamp is off...
... we disconnect the wires from the sensor terminals and, with the ignition on, short them.
The signaling device does not light up - we change the faulty lamp (see Replacement of a lamp of a signaling device of insufficient level of a brake liquid), and caught fire - the sensor is faulty. We change the faulty sensor assembly with the cover.
Unscrew the tank cap and remove the sensor.
We wrap the cover with a new sensor and connect the wires.