Abstract: A viscous coupling adapted to be positioned within the housing of a torque converter automatic transmission for use as a bypass of the torque converter and including an annular clutch lining secured to a side face of the outer peripheral portion of the coupling and a control valve assembly positioned in the outer peripheral portion to cushion the impact of engagement of the annular clutch lining with the confronting surface of the torque converter housing. The control valve assembly includes a rolling diaphragm which extends across the open end of an axially extending bore in the outer peripheral portion and is arranged to rollingly traverse the bore to successively cover or uncover a series of axially spaced leakage holes communicating with the main chamber of the torque converter and the bore.