So let's say I have set up the above drain system. So having the valve closed, I ran water through the top fillport and as I expected, the water split into two and filled the right side of the Y-fitting while water is pouring to the left as well.
I stopped the faucet and I removed the fill port on the bottom and opened the valved so the right side got emptied. Next I closed the valve and ran water again to the top fillport. Now it seems that the water will no longer go through the right side of the Y-fitting. What is this pressure that seems to be stopping the water from entering the right side of the Y-fitting? Have I completely lost my common sense here?