Presta valve on new tube won't inflate Just replaced a bad inner tube with a new, Presta valve tube. Unfortunately, I seem to be unable to get any air in the tube; I hear air leaving the pump and not going anywhere.

I tried investigating the usual suspects:


The pump is not sealed on tightly enough: Doubtful; several times I was able to lock the pump head so far down on the valve that it took considerable effort to remove it again.
The nut on the valve was closed: Nope. Opened and closed the valve several times, pushed down on the valve enough to hear a minuscule hiss of air. The valve is open.
Wrong pump type: Nope. Managed to inflate my other Presta valve tire just fine.


The only thing I can think of is that the complete lack of air pressure in the tube means there is nothing closing the valve, so the pump has nothing to depress and the sealing nut sits on the air opening. However, that sounds wrong to me.

Any ideas?
Perhaps its obvious, but maybe the tube has a leak? Is it an old tube that has perished and is just letting air out as fast as you pump it in?

Perhaps try taking the tube off the wheel, put it in a bucket/tub of water, and then pump.  Can you see bubbles somewhere?