The friction clutch between the prime mover and the change-speed transmission in the power train of a motor vehicle is actuatable by a slave cylinder cooperating with a master cylinder wherein the piston is movable by a clutch pedal or by an electric actuator. The casing of the slave cylinder is made, at least in part, of a plastic material and defines a plenum chamber arranged to receive pressurized fluid from the master cylinder by way of a conduit extending through a bell which contains the clutch and the slave cylinder. The annular piston of the slave cylinder surrounds the input shaft of the transmission. The plastic material of the casing of the slave cylinder can contain one or more additives which enhance its strength and/or influence its frictional engagement with the adjacent surfaces of the piston or of a lip seal which reciprocates with the piston. The piston and/or the lip seal can be installed in the casing to contact one or two metallic or plastic guide sleeves which are connected to or form part of the casing.