I am a lot more skeptical about venting a sump tank.  If the sump is vented and for any reason the mains don't gravity feed just right (it can and does happen - ask me how I know this :-) you can consume the fuel in the sump faster than it is gravity feeding from the main tank.  If the sump is vented, you will run it dry with lots and lots of fuel in the mains.  If you close the sump vent OTOH the sump level CAN'T go lower and the engine will "suck" fuel out of the mains to the sump (at least until one main empties and provides the sump with a "vent path" - you can ask me how I know this bit too :-).  I used to have my sump vent plumbed to the top of the cabin like Perry, and kept a dowel stuck in it to close it because I didn't want the sump to be able to empty before the mains.  One time I was driving along and smelled something and my wife looked over her shoulder and there was a fountain of fuel pouring out of the sump vent that had blown the dowel out.  She crawled into the back seat and stopped the leak with her fingers while I landed.  We drained a couple of gallons of gas out of the bilges. 

Now I have a schrader valve on my sump vent.  I take it off to allow the sump to fill all the way up and then cap it off again.  I know I'll NEVER fly with my sump vented, but like Perry I make sure the sump is full on preflight and if it isn't, vent it until it's full (and then cap it off).

Sump must be full and unvented to fly right ... Jim S.

Well, that's not my experience Jim. Over 500 hours now with a vented sump tank. But it may just be subtle differences that make our systems work differently. I wanted to use the stock Long-EZ fuel system, which is two lines, one from each main tank run forward to the cockpit where the L-R fuel selector valve is located. The selected main then runs back to the engine. I have a small one-gallon sump tank (welded up out of steel) that the selected main flow into by gravity. Two Mazda HP pumps are inside the sump tank, and fuel rail return is to the sump tank.

I think the Velocity system varies from this? No selector valve? Also no HP fuel pumps or fuel return to the sump?

I only had the sump tank unvented for maybe a month duration about a year ago.. I did not have any problems with the unvented sump tank until I left the plane sitting out in the sun all day away from my home base, where it's normally hangared. So I agree your idea of the shrader valve on the sump tank vent sounds like a good solution, making sure the sump is topped off before takeoff. I would also like to install a low-fuel-level sensor inside the sump tank, to give me maybe a couple minutes warning the sump tank is not filling.

Perry