Does the valve look like this? http://shop.easternmarine.com/images/product/thumbnails/sm_8651.jpg
If so, it goes after the leveling valve. Only valves I have used were pilot dumps and went between leveling valve and bags. Dumps the air out of the bags and blocks off the output from the leveling valve so it doesn't just vent. If this style was hooked up backwards I guess it would dump the leveling valve and not the bags thus emptying the tank.
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