Open system, that means fluid is always flowing thru, and is detored when you open the valve lever to the cylinder you want to acttivate, which in turn is still creating the loop, cause the fluid going in the cylinder is pushing it out on the other end and back into the system. I made this peice on a drill press, a good quility one. Just make sure you do a mirror image to the stock your using , very important. email me for more information or questions. traftonbell@wmconnect.com. The valve bank I bought is a 3 spool from prince , I paided $146.00. Plenty good for my use. And most of you guys also.