A hydrodynamic brake system wherein the extent to which the working chamber between the rotor and stator is filled with braking liquid is regulated by a valve which is adjustable by a measuring device having an overflow line communicating with two spaced-apart portions of the working chamber and containing two flow restrictors. The end portions of the overflow line are positioned in such a way that a continuous stream of liquid flows through the overflow line when the rotor is driven. At least one of the flow restrictors is adjustable so as to insure that the pressure of liquid in the overflow line between the flow restrictors is indicative of the real braking action of the system. Such pressure is used for adjustment of the valve to thus insure that the extent to which the working chamber is filled varies in response to changes in rotational speed of the rotor so as to guarantee a reproducible progress of the braking action.
