The principle of operation remains that of all breather tube filling systems, except that in this case the pressure inside the body of the pen is exerted through a hollow piston which is accessed from the bottom, which has an extended end part, but perforated at the center so as to be able to flow abundantly around the breather tube, which in this case is very long and comes close to the end of the pen. | The principle of operation remains that of all breather tube filling systems, except that in this case the pressure inside the body of the pen is exerted through a hollow piston which is accessed from the bottom, which has an extended end part, but perforated at the center so as to be able to flow abundantly around the breather tube, which in this case is very long and comes close to the end of the pen. |