Abstract: A warning circuit which generates signals in response to selected conditions of a vehicle hydraulic brake system. The brake system includes an engine-driven main source of hydraulic pressure, a relay-actuated auxiliary source of hydraulic pressure, a flow sensor for sensing the operation of the main source and a brake switch for coupling the auxiliary source with the battery in response to an operator brake application. The warning circuit includes a first transistor switch coupled with a warning lamp, with the ignition switch and with the auxiliary source. A second transistor switch is coupled to the ignition switch, to the auxiliary relay, to the flow sensor and to the first transistor switch. A third transistor switch is coupled to the ignition swtich, to the flow sensor and to the first transistor switch. The system actuates the auxiliary source upon failure of the main source during an operator brake application.