This video it clearly shows that you have to fully extend the pump or it won't work properly. Rather than getting one of these you can just easily build your own. I built one two years ago more along the lines of a ZX-2000.
I used 2" PVC Pipe with a 1 1/4" piston. Inline with and forming the part of the rod of the piston I put a single check valve. Since I didn't want any obstructions to my nozzle I use my finger to block the opening on the drawback and then move it aside when I want to spray. It's crude but effective. I have mine hooked to a five gallon water tank.