Abstract: Transmission unit for motor vehicles of the type which is provided with a continuously variable transmission (3), a reversing unit (2) and a number of clutches (16-18) to engage the reversing unit (2) in one or the other sense of rotation. At least one of these clutches (16-18) is controlled by a hydraulic pressure unit (25), with a piston (27) which can be moved in a cylinder (28), whereby the seal between the piston (27) and the cylinder (28) is an elastic sealing ring (32), The piston (27) is provided with at least one passage to let hydraulic medium escape under the influence of the centrifugal force, whereby the at least one passage (43) is located in the above-mentioned sealing ring (32).