It's real strange that when it is all assembled you can turn the input shaft freely but there is zero end play. is it possible you are not getting a firm grip on the input shaft and are not pulling it up enough to see the end play that is there? Wrap a thin strip of aluminum sheet around the splines on the input shaft and then grip it with some vise grips and use that to lift up the shaft to see if there is any end play. Remember that if you don't have the pump gasket in place when checking the end play, it will increase when you do install the gasket.