Automatic transmission with hydraulic system

The automatic transmission has a hydraulic system to operate and control a hydrodynamic retarder (6) which has a retarder chamber (9) constructed between a rotor (7) and a stator (8). The filling level of the retarder chamber is variable depending upon a position of a hydraulic step-up device in communication with the retarder chamber. The hydraulic step-up device is formed by a stepped piston (10) which defines a first piston chamber (11) with a larger effective area and a second piston chamber (12) with a smaller effective area, with the first piston chamber connected to the retarder chamber.