A rotating annular piston is hydraulically actuated to engage a rotating friction device, such as a clutch or brake, by introducing fluid pressure to an apply chamber for the piston. Disengagement is subsequently effected by exhausting fluid from the chamber, after which a centrifugal pressure head release valve in a wall of the chamber releases any residual entrapped fluid from the chamber in order to prevent undesired false actuation of the rotating piston that may otherwise occur due to centrifugal force acting on the residual fluid. The valve comprises a metallic ball subject to centrifugal force tending to move it radially outward from the piston axis but held in a tapered or conical seat of an exhaust port, thereby blocking the port, by fluid pressure supplied to the apply chamber. In the absence of that fluid pressure, however, the centrifugal force exerted on the ball unseats it and opens the port. By tilting the seat axis relative to the piston axis the ramp angle, formed between the piston axis and the portion of the seat radially outermost from the piston axis, may be minimized with a resultant maximizing of the rotational speed at which the piston may operate. The closer that the outermost seat portion is to being parallel to the piston axis, and thus perpendicular to the centrifugal force, the greater that force must be before the ball will unseat when there is fluid pressure in the chamber to move the piston in the apply direction.