here is a pic i put in my paint program

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 :)

I would say that the length of your pipe is too long after the siphon. the velocity of the water shooting down there is going to put alot of pressure on your intake, and with un-sealed joints, may be pulling in air. +1 to closing your valve 90% and trying again with a slower column of water in the pipe. Once you seal it up, you should be able to crank it open more.

Does the line in the tank suck any air at all?? If it gets any accumulated air in the over the top piece, it will loose siphon

did you prime by pouring? or prime by suction?

Ok well I don't want to run a pump for my overflow.
The pipe going down to my sump needs to be that long or it won't reach.
I primed it by filleing it with water and with suction niether worked.
It doesn't make any sense every other overflow I've seen on you tube works mint and for some reason mying won't. I'm just so confused this is my first overflow and I did it because I heard it was easy. Which it's not turnin out to be that way. Lol.

Can you get a siphon started at all? If you can, and it fizzles out after a bit, then chances are it's sucking air somewhere in the siphon loop. Either from the tank inlet, or the pipe connections. If the vent hole is not large enough, it may pull so hard it pulls a vortex in the tank end and sucks air that way?

i think it might be getting some air from the connections cause its not glued yet. but i finally got it to flow some water. i'm going to post a video on you tude and add the link to this page then everyone can see my issue. should have it up in the next ten mins or so stay tuned.

you tube
 

ok here is the link to my you tube video.

www.youtube.com/watch?v=hHeLRBUxwZc

Your vent hole is not large enouggh, Steve. The water level should only go down to the level of the pipe that your vent hole is in, Should never uncover the inlet like that. If the inlet sucks air, you will loose the siphon like how you are. keep increasing the vent hole bit by bit untill you find the diameter that works well.

It's hard to tell, but do you even have a vent just before the last 90 and the down pipe.