well i have the same problem but i screwed up. My elbow goes into the T fitting and then the actual T part goes to the sump and straight up is the open pipe. I ended up having to put an endcap on that to keep siphon.
OK the red section is whats wrong with mine. It allows the siphon to break without the end cap on it so to start the siphon I have a ball valve that has to be turned off. Fill up the siphon, put the end cap on, start the sump and open the valve. Also to feed the sump fast enough I have the slotted intake pipe upside down almost defeating the purpose, however I drilled a few holes above/at the water line and those skim the top. Meanwhile the underside with the slots allows for serious drainage into the sump without the terrible siphon/sucking noise. Basically when I built it I ended up putting the T junction on backwards which caused all my problems. I am going to rebuild it but as of right now it's siphoning just fine and with the float switch for my pump it's impossible for it to flood. What happens if the power goes out on mine the return line will siphon a bit (maybe 3 gallons) into the sump and trip the float which shuts the sump off until i take those gallons out of the sump.
Not ideal but it is now 100% impossible to flood, isn't making a lot of noise and is effectively skimming the top of the water and maintaining siphon and water level in the sump.