If your siphon break works properly, it should stop the siphon before the tank drains down to the level of the return pipes. Can your sump handle the water if they actually do drain down? I wouldn't want to be dependent on a siphon break to save me.
You may need to experiment a bit with how large a hole you'll need to create the siphon break. A large siphon needs a pretty large hole to stop it, otherwise it just behaves like a venturi!
Have you considered that you might not want that much flow through your sump?
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400 gal reef. Established April, 2007. 3 Sequence Dart, RM12-4 skimmer, 2 x OM4Ways, Yellow Tang, Maroon Clown (pair), Blonde Naso Tang, Vlamingi Tang, Foxface Rabbit, Unicorn Tang, 2 Pakistani Butterflies and a few coral gobies
My Tank: http://www.canreef.com/vbulletin/showthread.php?t=28436
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