Had a similar problem myself recently. After changing a tire, my valve stem leaked badly. Easy fix though. My problem was the shape of the gasket at the stem base. I had round gaskets which didn't quite fit into the "wheel groove" inside my Crossmax XL's. I found some online with square gaskets that are supposed to fit better, but I didn't want to spent $20 each. So, I used an Exacto blade, (any razor blade should work) and shaved the gasket to a more rectangular pattern. Hasn't leaked since-2rides. Give this a try. If you mess it up, you will have to buy a new valve stem. But it sounds like you might anyway. Nothing to lose. Hope this helps...