Make sure the cylinder is empty, you may have to dry-fire it 'till it dumps the last of the air through the barrel with a loud "hiss..!"
Take off the figure 8 clamp. Assuming it's the old-type filler, grip the cylinder behind the filling valve body, (AA part no. S490), with a strap wrench, or a leather belt and a pair of pliers. Fit a spanner on the male snap connector, S470, and the whole front end will come off in one piece. (be careful of the large O-ring, RN 234.) You can now look inside the cylinder.
Don't take the whole cylinder off if you dont have to.
HTH
Gus.