When I bought our dive op it came with 70 tanks, about a dozen of which had valves that needed repair/replacement. They have sat under my work bench for years, but I finally decided to replace the valves. It looks like these tanks had never been rinsed and after draining, the vaves are seriously hard to remove. I tried to loosen them up with a soft mallet, with no success. I put a pipe wrench on one an succeeded in removing the valve by tapping the pipe wrench with the soft mallet but scraped up the valve in the process. I would like to rebuild these valves, but I don't want to damage them in the removal process. What is the best way to get these buggars off? Thanks for any input.