I have a CS fountain pen with a converter. The converter sac has crumbled. I have gently attempted to remove the converter to replace the sac but to no avail. Is there a secret to removing the converter, direct pull, twist etc? Thanks for any advice.

Are you meaning a vintage pen with a pressac filler? If so, mine (on a 69 I think) just pulled off with a little bit of twisting to loosen it.

Yes that is what I have. I will try again, thanks