I just put this together the idea is the brass part (supplied by Mike C thank you again) goes where a spile would normally go and CO2 leaving the cask would be captured in the bag which is from a 10L cider bag in box (I will upgrade it to a 20 if it works) the other tap can be used either to let excess gas out if the bag fills, fill the bag with CO2 if it runs out or just let air in if the cask is almost done and the CO2 has run out. In theory I hope this can stop air getting into the cask therefore extending its life far beyond what it would normally be. I will be testing over the next week-10 days and will update on how well it works. Any comments and advise welcome and it anyone knows where a new brass bit could be made that would be great as Mike had it made as a 1 off.