A check valve piston filler works on a check valve principle that opens the infeed valve on the draw stroke then forces the draw side check valve closed while opening the discharge valve on the dispense stroke as can be seen clearly by the animation to the right.
The big advantage of a check valve filling system is it can self prime and draw product directly from a drum or other container without need for pumping or otherwise transferring product to another vessle. Simply drop the hose into the drum, adjust the fill volume and start filling product with excellent accuracy of +/- one half percent.
Check valve piston fillers work well with most any free flowing liquid (meaning it pours easily), but does not work well on thicker products or products with particulates in them as they can foul the valves.