Pinch valve construction

A pneumatically controlled liquid transfer system for delivering a liquid from a source to a delivery location, said liquid source being provided with a pneumatic cylinder for driving the liquid from said source, a pneumatically operated pinch valve of the make-before-break type and flexible conduit paths leading through the valve to the delivery location. The make-before-break pinch valve comprises a valve shell having two pistons movable laterally therein. The shell has a window. One piston has a yoke, each arm thereof having a passageway, one aligned with the other. A stop extends between the arms of the yoke. One of the flexible conduits is arranged through the window between the first piston and the yoke and the other flexible conduit is arranged also through the window but within the aligned passageways and between the piston and post. The second piston is spring biased to bear against the second conduit to place same in a normally closed condition. The first conduit is normally open. The first piston is acted upon by an external force such as air pressure to move same to a first condition wherein the piston pinches the first flexible conduit against the yoke, closing said conduit without overcoming the bias of the second piston to open the second conduit. Continued force exercised upon said first piston causes the same to move laterally within the housing to a second condition overcoming the spring bias of the second piston, to open the second conduit while the first conduit is retained in closed position. The second piston by virtue of its spring bias, will force the yoke back into its first position once the pressure on the first piston is relaxed. A manual override is provided operative upon the second piston to lock the same in open condition so that both conduits are open. This last mentioned position is utilized during down time of the apparatus concerned.