Actually my drawing was done in haste and my intake is in fact lower than the U tube. My flaw is the red section without an end cap it breaks siphon since its a straight shot right down to the sump. It's almost identical to the one you built except for the T junction mix up. It works great even with the flaw i built into it.
I'll get around to building a new one pretty soon, I'm just sick of fiddling with the sump/plumbing.
Megahaus I had the same problem, I was using a fluval 203 for a return pump lol. You will just have to find the right amount of throttle on the valve for now so your pump can keep up. Id say watch it for at least 20/30 minutes. Then go back and check it often until you can sleep well. I had to cap my open tube because I built it wrong. However if it is loosing siphon for no reason you probably have an air leak somewhere. I had a check valve in one of the caps sealed with some aquarium silicone and it came loose and would slowly cause a siphon break. So if it's slowly breaking siphon you might have a leak somewhere.
I think a flood is inevitable for any first time sump builders.
