Abstract: A piston is longitudinally slidable within a pressurized container to dispense materials from the container. The piston has a generally annular sidewall and a traverse barrier wall at one end of the sidewall and integral therewith to define a cup-shaped closure open at one end. An annular step is provided on the sidewall which divides the sidewall into two segments, an upper segment and a lower segment. The annular step is below and spaced from the barrier wall. The upper segment has a diameter smaller than the diameter of the lower segment and the clearance between the upper segment and the interior of the container is substantially greater than the clearance between the lower segment and the interior of the container.