D 150 Pickup V6-239 3.9L (1992)
Brake Fluid Pressure Sensor/Switch: Description and Operation
The switch activates a light on the dash panel to indicate that one of the hydraulic systems has failed. There are two basic types of switches. In some
earlier models, as pressure falls in one system, the normal pressure of the other system forces the piston to the inoperative side where it contacts the
switch terminal, causing the warning lights to illuminate.
On most models, the brake warning switch is of the latching type. If a pressure loss occurs in one side of the system, the piston in the valve is forced
toward the failed side where it latches in position and illuminates the warning light.