in general, a piston is a lubricated sliding shaft that fits tightly inside the opening of a cylinder. its purpose is to change the volume enclosed by the cylinder, to exert a force on a fluid inside the cylinder, to cover and uncover ports, or some combination of these. a rubber seal is sometimes used to keep the lubricate within the shaft. due to the constant motion of the machine this seal wears quickly and should be replaced with every servicing. if the seal should break during usage there can be disastrous long lasting consequences for the machine.