After everything that you have tried it sounds like you have an internal crack in valve.We have seen this many times over the last 25 years. There are plenty of salvage yards in Iowa that I would call to get a used header lift valve. I wouldn't think of spending $1200 for a new one. Check with Eiklenborg's in Aplington,IA or Teubel Salvage in Drakesville,IA. I would put the new o-rings in the used valve and you should be good to go. Hyper Harvest II