A hydraulic coupling of the temperature responsive type and particularly adapted to couple the engine to the radiator fan of an internal combustion engine. The specific improvement resides in a rolling dam adjacent the dump hole which connects the outer portion of the drive chamber containing the drive rotor with a shear liquid reservoir passageway. The dam rolls between two abutments carried by the housing cover so as to assume either of two positions with respect to the dump hole. One position of the dam relative to the dump hole accommodates relative rotation between the drive rotor and coupling housing in one direction, the other position of the dam relative to the dump hole accommodating such relative rotation in the other direction. The rolling dam automatically moves to the proper position relative to the dump hole upon either direction of relative rotation between the drive rotor and coupling housing.
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