I finally did it
After trying many methods and removing the glue (epoxy, araldit ?) the thing wouldn't disassemble.
So I took a hammer and a screwdriver and gently tapped in the "space" between the silver ring and the black cylinder until I could tear the ripped cylinder off. It was really glued so well that it wouldn't disassemble until I teared the whole parts away.
Here is a pic of the parts.
Many thanks to all of you !IMG-20121121-00214 - Copie.jpg