I believe what your looking at is called a push lock fitting. The way you disconnect them is to push in the collar {the top piece of the fitting} that the tube goes in to, and push in the tube at the same time. When the tube and collar both reach the bottom of the fitting you hold the collar down and that will release the tube and you can pull the tube out of the fitting. To reinstall the tube, all you do is push the tube in to the fitting and it will lock in. By the way, can you see any thing wrong with the valve ?