Steam is supplied to the steam brake cylinders from the locomotive boiler via a valve (the brake valve) which may have several fixed settings or be infinitely variable.
The piston in the steam cylinder is set in motion by the steam admitted, the piston rod transfers the brake force via a system of rods to the brake blocks of the vehicle, thus achieving the braking effect.
A steam brake acts directly and immediately; the sudden opening of the brake valve can easily result in overbraking by locking the wheels.
The operation of a steam brake therefore requires a great deal of experience from the engine driver.
In the latter case, a control valve linked to the vacuum system controls the admission of steam to the brake cylinder, so that the steam brake can be activated automatically in an emergency or if the train separates (the so-called 'automatic steam brake').