formed elastic ring 7b extends over the entire free axial dimension of structural part 3b between the annular faces 4b and 8b thus precluding axial displacement of the ring the ring has an outer diameter which is equal to the diameter of third structural 14b this is advantageous because the rolling diaphragm now rolls down partly along elastic ring 7b and partly along part 14b because of the constant diameter rolling down is now continuous without transitions this benefits the service life of the rolling diaphragm the convex side of rolling diaphragm 6c of the device shown in fig3 faces the piston 2c the rolling diaphragm being supported by liquid in liquid space 9c as a result of the suitably chosen surfaces of the annular faces 10c and 11c liquid space 9c has a constant volume at relative movement of rod 3y cylinder 1c a constant pressure difference is maintained across the diaphragm by means of a control unit not shown for example as described in french pat specification no 1 326 063 published 1963 corresponding to u s pat no 3 241 379 the rolling diaphragm 6c rolls down mainly along elastic ring 7c which again cannot move in the axial direction as a result of the limitation formed by annular face 8c in the device shown in fig4 the liquidsupported rolling.