the passage of the piston 6 through the housing 4 assumes one of its two end positions it is apparent that its path on this side is limited by a limit stop this limit is implemented by means of the appropriately formed internal contour of the housing 4 the bearing bush 41 is fixed axially by means of a limit stop disk 45 anchored inside the housing 4 the distance limit in the other direction occurs by means of a limit stop disk 46 inserted in the tie bar 5 which is limited with its face surface in this position of the piston 6 on which the limit stop disk 45 is limited in the housing 4 the protective cap 13 already mentioned above preferably consists of elastic material and is pot shaped in form wherein the cylindrical part of the protective cap 13 is provided with grooves 13 b extending up to their crown so that segments distributed over the circumference occur inside the cylindrical part these segments are provided with elevations directed inwards in the end section these elevations acts on the housing when the protective cap 13 comes in contact on the housing 4 as snapping segments 13 a since they are first widened radially so that when they hit the crown of the protective cap 13 the housing 4.