are obtained and intermediate parts of the end face of the piston 6 being at uniform height with one another serve as supporting surfaces for the rolling membrane 7 the vacuum ducts 19 as mentioned earlier may extend partially along the peripheral part of the piston but it is also possible of course to provide the piston with internal ducts which connect the peripheral central part of the piston with the end faces of the piston by establishing a connection between the central part of the end face of the piston and the vacuum chamber 13 it is ensured that the space between the piston top and the front rolling membrane 7 can be evacuated even if as generally is the case contact between the piston 6 and the rolling membrane 7 takes place in annular form along the edge of the piston top the contact between the piston 6 and the rolling membranes 7 8 is particularly difficult to secure when the piston reaches its extreme positions i e when owing to pressure variations in the line 5 it is moved into the vicinity of its end positions to ensure a satisfactory dampening of the movements of the piston 6.