Here are some pictures of a Yamaha Euphonium valve that sprung a leak.  What likely happened is that the valve was frozen and someone decided to free it up by driving some kind of implement (possibly a screwdriver) through the vent hole in the bottom valve cap.  Said implement went through the vent hole in the piston and punched right through the lowermost port (First picture), which happens to be the through port for the valve.  This had the result of causing an air leak unless the valve was depressed.  The second photo shows the hole as seen through the bottom vent hole of the piston.  I worked the metal back into place with a dent rod (Third photo) and then ran some solder into the hole to fill it (Last photo).  This is another fine example of why maintaining your instrument is so important.  If the valves had been oiled before storing the instrument (it belongs to a local marching band), the valve would not have been frozen and whatever was used to poke the hole in the port would have remained safely in a drawer someplace.