Abstract: A liquid dispenser in the form of a syringe having a barrel, with a piston valve/seal slidable in the barrel bore and having a preformed liquid passage hole extending through a slidable elastic main body thereof in which an elastic hour-glass shaped plug seal is removably disposed. A rearwardly extending post fixed on the forward end wall of the barrel bore effects rearward knockout removal of the plug seal from the preformed hole as a function of forward sliding movement of the piston valve/seal in the bore, and a liquid flow passage is formed in the post to enable passage of liquid therepast. The piston valve/seal main body has a plug-containment chamber which is larger than the plug and enables by-pass liquid flow past the plug after dislodgment of the plug into this chamber.