I have found many different tube styles when shopping for tubes. Currently I have a tube with a very long valve stem on the front and a short stem on the back. Both are hard to fill, and if I get a flat I doubt that my hand pump will be able to do the job. They lose air as I attempt to pump the tires up. Are there certain brands that have "good" tubes, or am I buying the wrong tubes? I have purchased at my LBS and from Nashbar. Thanks.