Repairs can be made without cutting into a tube if care is taken. First unscrew the plastic nut from the valve body. The valve can then be pushed inside the tube. Feel through the tube fabric (Fig 1); turn the valve body upside down, so black rubber diaphragm is at the hole in the tube.
Press a finger in to find the white plunger in the valve body. (Fig 2). Press on plunger and unscrew the diaphragm.
Keep finger pressure on the plunger and carefully turn the body back to its correct position. Remove the white plunger and spring (Fig 3).
To insert new diaphram reverse the above steps.