I was wondering if you ever tried to remove and reseat the inner spring-loaded pin from inside the valve stem.
Sometimes a little dirt or gum gets wedged in there while filling up the tires. By using a valve stem tool, you can unscrew the inner workings of the valve stem, view it, resecure it tightly, and refill it.
It has happened to me a few times on different car and bicycle tires. A three dollar valve stem tool saved me several times... You simply insert it into the stem and unscrew the spring loaded pin valve mechanism. Clean it up and re install it..
Any of the autoparts chains or Walmart will have them.
Edit: here is a
Video Clip for the tool.
