Overflow, breaking siphon!

[MrGemini]

Ok so I have. 60g rimless cube. So far all is well but I need to stop siphon when return is shut off. I believe I got the issue taken care of the back siphon on the return, I put a check valve on it to stop, but now how do I stop the siphon on the overflow aka down pipe. Any suggestions are welcomed or should I let be as it's not that much that comes down after pumps off but it does get close to the top of the 10g sump.

 

[steve&mari]

If it's not flooding sump it should be ok. That's where people normally adjust the water levels in chambers. So when powers lost it can handle the tank draining to bottom of overflow teeth.