The spring-loaded composite brake chamber consists of two relatively independent air chambers combined together. The front chamber has the same structure and function as a general brake chamber — it is the actuating device of the braking system, converting input air pressure into mechanical energy transmitted to the wheel brakes.

The rear chamber differs from a general air chamber in that it contains a powerful spring. When the chamber is not working, the spring is compressed by the input air pressure and its potential energy is stored for emergency use.

When the service brake system or the vehicle's air circuit fails, the pressure in the spring-loaded chamber drops rapidly (or can be released via a control valve). The spring then releases its stored energy and, through a conduit, pushes the piston rod of the front chamber, causing the brake to engage and slowing or stopping the vehicle.