Unscrew the barrel from the nib and thread connection. Remove the screw holding the end knob to the plunger. Remove the plunger. Pry out the O-ring with a jeweler's screwdriver.
Clean the barrel with water and dry it. Put a light layer of silicone grease on the new O-ring and push it into the O-ring slot.
Reattach the end knob and secure it in place with the screw.
Pull the sac protector off the sac. Use a drill bit with a diameter smaller than the sac protector to clean out the residue on the inside of the protector.
Pull off the old sac and replace it with a new one. Dust it with talc, then reattach the sac protector. Reattach the barrel to the thread on the nib.
Unscrew the nib assembly from the ribbed section. Remove the point seal with a jeweler's screwdriver. Insert a new point seal coated with silicone grease. Reattach the nib to the ribbed section.