Hydrodynamic retarder device, for connection and disconnection to a power-train in a vehicle, wherein the retarder device comprises: - at least one shovel equipped stator, which, together with at least one shovel equipped rotor, forms a shovel system with a workspace for receipt of an aqueous working medium, - a retarder circuit connected to the workspace, in order to control the inflow of working medium to the workspace, wherein the retarder circuit is arranged to be connected to the vehicle's ordinary coolant circuit, and - a clutch device for connection and disconnection of said retarder device to/from said power-train, characterised in that - the retarder device is arranged to ensure that the pressure in the workspace is decreased to or below the vapour pressure for the working medium, in order to vacate the workspace of liquid working medium in connection with a disconnection of the retarder device from the power-train by way of: i) initiating a valve element arranged in the retarder circuit to shut off the inflow of working medium to the workspace, while allowing an out-flow therefrom, during a first time period when the retarder device is connected to the power-train while the rotor continues to rotate, and ii) to subsequently activate a negative pressure generator connected to the workspace, during the continued rotation of the rotor, - to keep the valve element closed and the workspace in an evacuated state during a second time period, when the retarder device is disconnected from the power-train, and during a subsequent, third time period for reconnection of the retarder device to the power-train for subsequent braking action. - at least one shovel equipped stator, which, together with at least one shovel equipped rotor, forms a shovel system with a workspace for receipt of an aqueous working medium, - a retarder circuit connected to the workspace, in order to control the inflow of working medium to the workspace, wherein the retarder circuit is arranged to be connected to the vehicle's ordinary coolant circuit, and - a clutch device for connection and disconnection of said retarder device to/from said power-train, characterised in that - the retarder device is arranged to ensure that the pressure in the workspace is decreased to or below the vapour pressure for the working medium, in order to vacate the workspace of liquid working medium in connection with a disconnection of the retarder device from the power-train by way of: i) initiating a valve element arranged in the retarder circuit to shut off the inflow of working medium to the workspace, while allowing an out-flow therefrom, during a first time period when the retarder device is connected to the power-train while the rotor continues to rotate, and ii) to subsequently activate a negative pressure generator connected to the workspace, during the continued rotation of the rotor, i) initiating a valve element arranged in the retarder circuit to shut off the inflow of working medium to the workspace, while allowing an out-flow therefrom, during a first time period when the retarder device is connected to the power-train while the rotor continues to rotate, and ii) to subsequently activate a negative pressure generator connected to the workspace, during the continued rotation of the rotor, - to keep the valve element closed and the workspace in an evacuated state during a second time period, when the retarder device is disconnected from the power-train, and during a subsequent, third time period for reconnection of the retarder device to the power-train for subsequent braking action.