Pressure Differential Switch
The pressure differential switch illuminates the red brake warning light when there's a difference in pressure between the two hydraulic circuits of a dual master cylinder. The light informs the driver that there's a problem with the base braking system. When pressures are normal and both sides have equal pressure, the piston is centered by two self-centering springs. The light is not illuminated when the piston is centered.
When there's a loss in pressure in one of the two circuits, the piston moves toward the side with the lowest pressure. This closes the switch and completes the circuit to the red brake warning light. The loss in pressure can be caused by an external leak or aerated fluid. The brake light warning switch shown is located inside of the combination valve along with the metering and proportioning valves.