Abstract: A gas compressor or motor of the elastic bellows type is provided with improved facilities for maintaining the diaphragm in contact with an actuating member that reciprocates the roll sock between its expanded and compressed positions. An auxiliary piston-cylinder set is disposed within the machine housing to establish a pressure differential between the two sides of the roll sock as the actuating member retracts so that the difference in air pressure maintains the surface of the roll sock firmly in contact with the actuating means. The resulting prevention of collapse of the roll sock permits a full intake of operating air or other gaseous fluid during the suction stroke, and greatly improves the efficiency of the machine.