Abstract: A liquid level sensor for sensing the levels of brake fluid in the master cylinders of a vehicle having probes for insertion into the fluids of the master cylinders and threshold detector circuits, associated with the probes and having an integrator coupled to the probes for providing a signal to activate a warning mechanism such as a lamp when the level of the liquid in either cylinder falls below a predetermined level. A protection circuit is coupled to the threshold detector circuits for protecting them against excessive transient voltages or a reverse supply potential. An additional circuit is coupled to a monitoring system which monitors the differential pressure switch which is activated in response to a difference in pressure in the two master cylinders. A self-check circuit is also coupled to the threshold detectors to activate the warning lamp in response to a signal from the ignition switch.