Originally Posted by AndreyT

Um... The value that ensures one-way air flow is located in the pump. As long as the pump head is attached to your tube valve, the tube valve is supposed to stay open the whole time. It is not supposed to act as a "valve" when you are pumping up your tube.

So, if you observe this "rubber lung" effect when you attempt to inflate your tube, it is your built-in pump value that failed, not the valve core in your tube.