A hydraulic clutch which absorbs little power when disengaged but provides maximum torque transmission almost instantaneously when engaged, employs a variable volume reservoir (17) to and from which the hydraulic working fluid is transferred for the different operating regimes. When disengaged, the volume is a maximum and vice versa. The torque converter (11) has hydraulic fluid delivery (22) and discharge ducts, connected to a pump (25), cooler (26) and the reservoir. A valve (28) controls the pressure applied to a piston (19) or membrane in the reservoir and may be actuated by the engine throttle control. When disengaged, pressure in the converter forces the working fluid out against only atmospheric pressure on the piston (19). Rotation of the valve (28) connects a higher pressure source (31) to the piston and returns fluid to the converter.
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