A rotary valve including a movable valve seat which is caused to be pressed against a rotary valve plug when the latter is positioned at its full close position or at its full close and full open positions and to be moved away from said rotary valve plug during the time the latter is being rotated to open or close the valve, by means of a fluid-pressure-means which is automatically actuated by the operation of a driving shaft for rotating said valve plug.

http://www.freepatentsonline.com/3901474.html