Abstract: A piston unit is disclosed which includes a pressure-compensating device wherein pressure variations in a product line are dampened by subjecting a side of the piston (6) remote from the product line (5) to a variable air pressure. Rolling membranes (7,8) are positioned on both sides of the piston and enclose between them a chamber (13) which is connected to a source of vacuum (14) so as to secure contact of the membranes with the piston. The vacuum is distributed in the chamber (13) with the help of a vacuum line (15) with enlarged outlet area, and is distributed over the end faces of the piston (6) by way of a network of ducts (19,20) in the piston.