I encountered this pose while searching for issues related to my leaking ATO screw valve, which only stopped leaking when slightly tightened, cutting off the water flow to the float valve.
After tinkering with the valve over a few days I found the Oring placement is key to ensuring proper function.
1. First, fully unscrew the valve and inspect the O-ring's position. If the O-ring is located at the bottom of the shaft (see picture 1), it suggests a poor seal.
Picture 1 (Incorrect placement)
2. Dry the shaft and place the Oring at the tip of the shaft as in picture 2
Picture 2 (correct placement)
3. Insert the shaft and the Oring will seat in the correct position. See picture 3 where the black Oring is visible through the translucent plastic. Tightened the crew slightly.
Picture 3
4. Connect up the ATO reservoir and loosen the screw valve until you see a stream of water.
Picture 4
5. Push the ATO feed hose into the valve whilst the water is trickling. This ensures the right flow is feeding the float valve. Check there is not leake.
Picture 5
And Whala.... all done.
I hope this can help anyone with the same issue as me.