A vented piston (1) comprises a plunger (2) having a front side and a rear side, a venting passage (4) extending between the front side and the rear side, and a cap (7) located adjacent the front side. The cap is resiliently-biased by a spring (8) towards an open position in which air can pass through the venting passage from the front to the rear side of the plunger, and is movable from the open position into a closed position in which the cap closes the venting passage. The piston may be used in a storage chamber (10) of a dispensing system whereby air can be removed from the storage chamber by moving the plunger in the direction of a substance (12) stored in the chamber.