If you don't have any visible signs of pitting on the value or seat, I'd rebuild ($13) or I just found a new petcock assembly for $55 (
http://www.metricpartsoutlet.com). When the vacuum diaphragm started leaking on mine from the weep hole in the back, I did a rebuild with Kawa parts and went ahead and did the Bergmen conversion to a manual petcock. This won't eliminate the possibility of it leaking from the diaphragm but I really like the idea of being able to completely shut the gas off.