The device comprises a cylinder (1) which is divided in two variable volume chambers (3, 4) by a piston (2, 2'), which has conduits (5, 5') for the passage of a fluid from one chamber to the other, and spring means (15) for pushing the piston in a predetermined direction, tending to cause a pressure increase in one of the chambers; the piston is mounted on a rod formed of two aligned rod elements (7, 8), with means (9) for bringing closer and separating their adjacent extremities; it comprises means for providing a controlled fluid leak through the conduits (5, 5') of the piston, said means consisting of a groove (25) provided in the piston (2') or washers (12, 13) which can be deformed due to the existence of an undercut (33) in one of the rod elements. It provides for the automatic return of the seat and/or the back of the seat to a predetermined position when the passanger leaves the seat.