I've had two 700c tubes develop punctures right at the base of the valve stem. I am using a Silca pump. It goes onto the stem easily, but is a little difficult to pull off. However, I don't think I am yanking it side-to-side when I am pulling it off the stem. I was wondering whether putting a little grease on the stem would allow the pump head to go on/off easier, but am afraid of air leakage. Anyone have similar problem? Any ideas?