Abstract: A control valve of a hydraulic coupling controls pumping through a casing inlet port by a hydraulic pump and/or through a casing outlet port to control coupling between two rotary members such as by operation of a clutch. The casing is connected to one of the rotary members while a pumping component embodied by an impeller is connected to the other rotary member and meshed with an internal ring gear having one more tooth than the number of impeller teeth to provide sufficient pumping capacity so that the pump can act as a brake while still having relatively constant pumping pressure. A pressure port plate creates a hydraulic flow path for a gerotor pump and maintains pressure equalization about the gerotor pump. The pressure/torque controlling groove formed in the grooved port plate creates an efficient use of space, and the dimensions and cross sectional area of the groove can be changed easily to produce different pressure/torque curves.