Hehe, at the moment Im building a fun little tank with 4 of these surge devices on them. =D. I had the same problem; the drain would just equalize with the pump rate. So... Whatcha have to do is have the flapper, then a U bend underneath the flapper, and then bend it back towards the tanks slightly lower than the flapper. Lemme see if I can ASCII this...
_ _ O - flapper
| | |
| ---
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| - to tank
Hope that helps,
tommarow I will post some pictures