I have Velocity Deep V's which require tubes with long presta valves. The spare tubes use regular length presta valves so I bought a valve extender that screws onto the presta valve to make it long enough to use with the Deep V rims.
Well today I had a flat so I swapped the tube and added the valve extender. I removed the nut that holds the valve against the outside of the rim and put the extender on and finger tightened as much as I could. I made sure to loosen the nut on the valve so I could inflate it before I put the extender on. The extender itself doesn't have a tightening nut and is really just a long cylinder that threads onto the valve. When I started pumping, I couldn't inflate the tube more than maybe 30-40lbs/in pressure before the air would would leak around the seal between the pump and valve extender. This wasn't enough to ride on so I ended up walking the rest of the way to work.
Does anybody have any experience with these extenders? What am I doing wrong? Any insight would be appreciated.