A heavy vehicle is a road vehicle weighing more than 3.5 tonnes, assigned either to the transport of goods (truck, semi-trailer, road train) or to transport people (bus, coach, trolleybus). These vehicles often use air braking, the power of which is controlled by compressed air. Hence the need to study and analyze the braking system and the importance of having an emergency circuit.

The single control valve is used on heavy-duty vehicles. This tap performs two functions:

- Classic main braking

- Emergency braking taking place automatically as soon as there is a drop in pressure in the main circuit.

The two circuits are completely independent and the failure of one cannot affect the other.

Normal braking: when the control lever is actuated, it causes the piston to move and the valve to open. The front and rear brake cylinders are then supplied with air at pressure P0 coming from the main reservoir. When released, the lever is no longer actuated, the piston returns to its initial position and the valve closes. The pressure will escape to the outside (Atmospheric pressure).

Emergency braking: If following a failure in the main circuit (rupture, leak, etc.), the pressure in this circuit can only reach a P value less than P0, normal braking is no longer carried out correctly. As a result, the driver continues to operate the lever, causing the assembly to move and open the emergency valve. The pressure is sufficient to make a tilt which will allow the pressure to pass. The AR cylinders are therefore supplied with air at normal operating pressure from the emergency tank. The pressure is greater on release when the emergency valve is closed than the pressure in, which will cause the emergency valve to open and the pressure to escape.