A pressure differential switch (sometimes also called a distribution block or tee) is designed to detect a difference in brake system pressure between the front and rear portions of the braking system on a car equipped with a dual master cylinder. The switch assembly contains a sealed piston (or pistons), an electrical terminal, and sometimes springs. The piston moves from a centered position when the fluid pressure on one side of the piston is higher than the other, usually as a result of a fluid leak. When the piston moves off-center the switch is closed, completing a circuit to chassis ground and lighting a warning lamp in the instrument cluster.
Pressure differential switches can be used as stand-alone parts or as part of a larger combination valve. If you see an electrical switch on a valve you can be sure that you're looking at a pressure differential switch.
