The invention relates to a piston (4) with a sealing element (7, 7a-b) for slideably sealing a container (1) with an essentially cylindrical inner surface. The sealing element (7, 7a-b) comprises an inner portion (7b) aligned against the piston and an outer ring portion (7a) which extends in a ring shape around the circumference of the piston and is arranged to engage the inner surface of the container (1). The outer ring portion is arranged to flex inwards towards the centre axis of the piston (4) when the piston is displaced inside container (1) in a first direction and the outer ring portion is arranged to flex outwards towards the inner surface of the container (1) when the piston is displaced inside the container (1) in second direction being opposite the first direction. At least a first ring shaped groove extends in a ring shape on a first side of the sealing element (7, 7a-b), said groove being arranged between the inner portion (7b) and the outer ring portion (7a) of the sealing element.
