A very easy fix is to clamp a short length of hose onto the bottom nipple with a plug clamped in the hose.
You can get fancy and route the hose to somewhere handy, with a cut-off valve up top, close to the seperator.
Or, you can dis-assemble and re-build the seperator that you have.
Or, and by far the best alternative, mount a GOLDENROD fuel-filter/sediment-bowl where the unit you have is mounted, and use the water seperating cartridges in it.
http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_910811_910811
You can get a Goldenrod at Tractor Supply, ACE, or just about any farm-supply store.
I am a firm believer in the Goldenrod units.