Abstract: Two closely juxtaposed coaxial disks, one fixed and the other rotatable, are formed with confronting annular grooves partitioned by slanting vanes into coextensive pockets designed to decelerate the rotation upon the admission of hydraulic fluid into a chamber formed around the rotatable disk by the fixed disk and a brake housing secured thereto. The fixed disk has a limitedly rotatable annular insert which carries at least a part of each stationary vane thereof and can be moved between a working position in which its pockets are mutually isolated and an idling position in which these pockets intercommunicate either directly or through an external space. The annular insert may be urged into the idling position by a set of springs whose force is overcome, upon the production of hydraulic fluid, by the reaction torque exerted upon the stationary vanes.