Subject heading pretty much says it all. My tire has a presta valve; if I unscrew the top and push it down, nothing happens. My pump won't put any / much air in. I just want to replace the tube, but there's no way I can fit tire levers in there until the tube is deflated. What other options are there for deflating tires? I'd rather not stick a knife through the tire and pop it as the tire itself only has a few hundred miles on it. Can I cut the valve stem? Would that deflate the tire?