Where did you break the valve stem? At the base? At the valve?

If it is breaking at the base of the stem, your problem may be that you are running presta in schraeder holes (too big). The tube sneaks out and eventually gets pinched. I lost a lot of tubes before I figured that out. You can fix this problem by installing valve stem adapters http://wheelsmfg.com/products.php?ca...restastemsaver .