Abstract: A tank for storing fluid materials includes a piston having a channel, and an annular seal secured to the channel. The annular seal projects outwardly from the circumference of the piston. The piston further includes a plurality of radially arranged, spaced-apart tensioners that stretch the seal into sliding engagement with the interior surface of the tank. As the piston slides through the tank, the tensioners independently move radially inward and outward to compensate for shape-wise imperfections of the tank, such that the fluid materials extrude through the opening of the tank without bypassing the annular seal.