Tony,
Yes, the "T" is placed vertically and with pvc ends on top (remains uncapped) and bottom with the valve.
It will also tell you if that was your problem, by covering the top with your hand....you will see the water level actually go down if siphon is being created.(we hope it is after doing all this
)
I dont believe that this would be the best advise if your replacing the ballvalve with a larger diam gate valve,and placing this gate valve directly onto the first portion of ther skimmer (minimal plumbing before it).
Your best solution would be to actually run the skimmer output directly at or above your sump average waterline, or directly in your sump at the same water level,running just the gate valve on it.
Good luck,