Question, perhaps I'm just not reading this right, does the air CONTINUE to escape after the pump is released from the tube? If so it sounds like it may also be a stuck or loose valve core. You can get a valve core tool nearly anywhere, I got mine with a pack of replacement cores. Place the tool inside the stem, interlock the core and try and tighten it gently, if it moves much at all then it was loose, try pumping again. If it holds now it's fixed, if it leaks again try replacing the core. Unscrew the core with the tool and remove it and replace it with a new one. If it still gives you issues then it must be damaged somehow. Could it be a tear in the tube near the stem?