Overflow box with "U" tube problem

[evilvelle]

I hate my overflow box wish I could do it all over and drill tank but oh I have to deal with it. I am having a problem when I turn my return pump off to feed and turn back on tank will overfill. I have my return with a T and a valve to control how much flow goes to tank and diverts it back sump. My U tubes have no air all seems ok cant figure it out any ideas?

 

[emooring37]

what brand overflow? I have two lifereef ones and they never once lost siphon....the design makes a difference. Or a pic would help troubleshoot...