This foot-operated valve (Fig. 10) applies air to operate the brakes. The amount of air delivered to the brakes is regulated by the driver according to the distance the treadle or brake pedal is depressed. Releasing it exhausts air in the service brakes through its exhaust port.
These valves are made in overhead styles with a foot pedal hanging down, or a floor-mounted version with a foot treadle.

Figure 10. Dual-circuit foot valve