As Sphelps mentioned, he used two 3/4" drains similar to yours to drain about 600 GPH. How much do you intend to run through yours?
While I don't think it's a good way to drain a tank, I've seen all kinds of variations of what you are doing, so let's go ahead and trouble shoot it so you can decide if it works for your application.
1) Attach a hose to the intake in the tank and run water through your drain for a minute to assure that all of the air is purged out of the pipes.
2) Run it with a modest pump (500 GPH) and let us know what happens.
- does the tank water level rise over the intake/overflow the tank?
- does the drain quickly siphon for a minute or two and shut down?
- does the drain make a toilet flush sound?
3) Play around with the valve, starting with almost completely closed and slowly work it open until you lose siphon.
4) Play around with the return pump and slowly increase the pressure/volume until the siphon can't keep up.
5) After you get it "tuned" to operate properly, do a simulated power outage and make sure it starts up on its own.
Are you using this with an overflow box for surface skimming? If not, you aren't getting efficient skimming and you will need a strainer to keep critters out.
Do you have a fail safe system if a snail clogs the pipe? Where does all that water in your sump go in the event of pump/power failure or pipe blockage?
Sometimes a cheap overflow ain't so cheap
