The original post is almost 5 years old, but since there is 2 identical O-rings in there, doing the same job, where one is completely destroyed, and the other is undamaged, it looks to me being a user error where the adjuster screw has been adjusted to far out, and has lost sealing surface. If you look at Ernest Rowe`s video, and compare it to the picture, you will see the O-ring is plit in halves, just like he shows will happen if you turn the adjsuter screw to far: https://www.youtube.com/watch?v=ubNmXuZ_iI0