To enable the driver to apply the service brakes by pressing the brake pedal, a foot (treadle) valve is located underneath the pedal. This valve has two supply (input) ports (one from the primary air tank and one from the secondary air tank), two delivery (output) ports, and an exhaust port. When the driver presses the brake pedal, the foot valve opens the delivery ports, and compressed air leaves the delivery ports to activate the service brakes. When the driver releases the brake pedal, the delivery ports close. The air in the brake chambers and hoses is exhausted to the atmosphere, releasing the service brakes. (Note: In many modern CMVs, only some of that air is exhausted through the foot valve’s exhaust port. Some of it is exhausted through the exhaust ports of other valves.)