vulcanized lip sealing rings in the figures pressure oil feed lines are designated with the reference signs 15 within the framework of an additional form of the invention and with reference to fig2 the spring elements 9 10 at the two end positions of the actuating piston 6 are designed or arranged to be preloaded in order to achieve a defined partial resetting in the example shown in fig2 the spring elements 9 10 designed as disk springs are arranged to be clamped and thus preloaded in the embodiments in accordance with fig1 and 2 the actuating piston 6 is pre assembled or installed in a casing 12 together with the sealing elements 11 and the spring elements 9 10 whereas the pre assembled casing 12 is pressed into the housing 2 and is axially fixed on the mounting side by an additional component or a retaining element in accordance with an additional form of the invention that is the subject matter of fig3 a partial resetting of the actuating piston 6 is achieved not by spring elements as shown in fig1 and 2 but through elastomer buffers 13 14 whereas the elastomer buffers and the sealing element 11 which may be designed for example as a vulcanized lip sealing ring preferably designed in one piece with the example shown in fig3 analogous to the embodiments under fig1 or 2 the actuating piston 6 is pre assembled or installed in a casing 12 together with the sealing elements 11 and the elastomer buffers 13 14 in addition with the embodiment.