Clean and grease all selrings with food-silicone-grease.

When reassembling start with the part as shown on the first photo. Don't forget the sealring on both ends !

Second and third picture show the ready assembled valve: Push the white part with the spring together and insert it in the valve-block. You see the notches, they only fit in one direction. :) I now understand that my camera is bad, I hope I may buy a better one.