According to the pressure change of the throttle valve action tube, the action valve has three action positions: relief, braking and pressure maintaining.
(1) Mitigation effect. Relief position When the pressure of the action tube under the action piston drops, the action piston drives the hollow valve stem downward under the pressure of the brake cylinder and the relief spring. The upper end of the hollow valve stem leaves the bottom surface of the air supply valve and opens the exhaust valve port The compressed air from the brake cylinder above the piston is discharged to the atmosphere through the center hole of the hollow valve stem, and the locomotive is in a state of relief.
(2) Pressure-holding effect. When the pressure of the action tube below the piston increases, the action piston moves up with the hollow valve stem. First, the upper end of the hollow valve stem is closely attached to the bottom surface of the air supply valve to close the exhaust valve port. Then, the air supply valve is opened to open the air supply valve port.
(3) Braking effect. In the braking position, as the pressure of the brake cylinder continues to rise, the pressure above the acting piston gradually increases. The acting piston gradually moves down under the action of the pressure of the brake cylinder above it, the relief spring and the spring pressure of the air supply valve. The opening of the valve gradually decreases.