Originally Posted by
cyccommute
That would depend on the kind of valve. Schrader valves have to be held open to get air in the tube. Presta valves are check valves with a low cracking pressure. Air goes in and the valve closes once there is a pressure differential between the air supply and the pressure in the tube.
True. I assume we are talking about a Presta valve here (although this makes no difference if a bicycle-specific pump is used).
Originally Posted by
cyccommute
A Presta valve is supposed to act as a valve when air is being added.
Absolutely not. As you correctly stated above "valve closes once there is a pressure differential between the air supply and the pressure in the tube". But because of the built-in one-way valve in bicycle pumps, once you start pumping there will be no pressure differential between the tube and the pump hose. I.e. there's no pressure differential across the Presta valve. As long as the pump head is securely attached, Presta valve remains "free floating", "unloaded", "loose" or permanently open. Some bicycle pumps (e.g. some Lezyne models) are actually
designed to apply direct mechanical pressure on Presta valve's head to positively ensure that it remains permanently open. But most pumps just rely on the absence of pressure differential across Presta valve.
The pressure differential exists across the one-way valve built into the pump. There's no pressure differential across the tube valve. That is if the pump is working properly.
If your Presta valve actually "acts as a valve when air is being added" using a regular bicycle-specific pump, it means that there is something wrong with your pump: either the pump's built-in one-way valve is broken, or there's a leak somewhere on the path from the pump's valve to Presta valve.
A small leak is typically OK. But if the tube is acting as a "rubber lung" (per OP's description), i.e. all air pushed in on forward stroke gets evacuated at backstroke, it immediately means that the pump is hopelessly broken. The valve core could be broken as well, but that would be a secondary problem.
To illustrate it even further: if the pump is working properly, you can completely remove the core from a Presta valve. And you should still be able to successfully pump up the tube to whatever pressure you want. (It will be difficult to keep air in the tube once you detach the pump head, but that's a different story.) There should be no "rubber lung" effect even with the core completely removed.