Overflow - What am I doing wrong - Fixed

[rocketlily]

I have set up a 50 gal tank with a Marineland overflow and a Quietone 4000 return pump. We have put the tubing in the slot, held a hand over the return hose to get the water in the outer box over the plastic U end, sucked on the tubing, removed the air from the U, turned on the pump, got the siphon going, then all the water drains quickly from the outer box breaking the siphon.

Not sure why the outer box would all of a sudden drain so fast. Please help.