i think you are almost getting it. You need to remember that the T area is not your exit point. It's simply a "loop in the hose". the exit to your pipe is in the sump, not the T. If yours is working it's because of an air bubble in teh "t", leaky connections allowing air in, ie the air line you tapped in, or the flow is slow enough that the siphon is broken from air entering the exit of the pipe. Otherwise, an airtight version of what you pictured will drain until the siphon is broken from the intake.