A relatively movable piston and cylinder are joined by an elastic sealing element which engages the piston and cylinder at spaced locations and at least partially defines a working chamber. The sealing element bears against a bearing surface via a lubricant space, and means are provided for feeding lubricant between the bearing surface and the sealing element. Part of the bearing surface comprises a transition surface section extending oblique to the axis of the cylinder. The lubricant feed mouth opens to the bearing surface at a predetermined location relative to the transition surface section of the bearing surface.