<p>A piston-type dosing valve is set forth that provides a hermetically sealed chamber behind the diaphragm which separates the aseptic portion of the valve from the mechanical portion of the valve. An external vacuum source is connected to an opening in the chamber thereby maintaining a constant negative pressure within the chamber. Any breach in the diaphragm will be detected by a pressure sensor located within the sealed chamber. In one embodiment, visual inspection for the presence of liquid product in the sealed chamber can be accomplished through a window in the dosing valve housing located adjacent to the diaphragm and sealed chamber.</p>