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Dual braking systems must have a pressure-differential valve in order to remain separated, plus inform the driver when theres trouble. This valve is affected by brake pressure difference between front and rear brakes. A loss of pressure in the rear brakes slides the valve piston to one side, closing the switch and illuminating the BRAKE light on the instrument panel. Less than motivated mechanics have disconnected these switches through the years rather than do a differential brake bleeding to center the piston.