Abstract: A hydrodynamic braking system with a retarder including a rotor arranged in a rotor housing and a stator arranged in a stator housing, whereby the rotor and the stator together form a working space. The rotor is axially movable relative to the stator, from a first position (braking operating position) to a second position (non-braking operating position), and vice versa. The axial distance between the rotor and the stator in the non-braking operating position is a multiple of the distance separating them in the braking operating position. According to one aspect of the present invention the rotor housing includes an outlet that is located at a distance from the axis of rotation of the rotor and is open toward the rotor in the non-braking operating position such that the operating medium collected by the rotor is conveyed outside the working space through the outlet.