And this here is a good one: It drives many people nuts. I did a lot of internet research, and even those, who managed to open it, describe the process that badly and without pictures that you need a lot of fantasy to follow their drift: Use a flat screwdriver to push the "black plastic ring" (it is more than a ring, but that is all you can see) into the brass fitting, only then pull on the black hose (possibly in and out movement).