Ya Dale after much thought I kinda thought of that as well.
The only way that would work is if the return line came up above the water level first. and the "T" was placed above the water line.
Some other suggestions I've had have been
true union check Valve (that way I could clean it)
mechanical valve with the no power or default position closed.
I'm sorta leaning back to the Check Valve, If I could clean it and get one of very high quality it may just do for my purposes.
now to find a local source
http://www.marinedepot.com/aquarium_...tings_2.asp#cv
J