The reason why this Parker 21 Super’s disassembled is because I was trying to find out why it wasn’t drawing ink. Air wasn’t leaking from anywhere above the connector so I guessed the problem lay with the sac. So I heated the sac guard, pulled it off using section pliers (which, I later read, Marshall and Oldfield advise against), and discovered the problem — a slit in the pli-glass sac.
I was about to reach for a silicone sac when I recalled I had a brand-new Hero 336 lying around. Wasn’t that pen based on the “51″? Couldn’t its sac fit the 21 Super…? Only one way to find out —
The fit turned out to be just right 
(And an added benefit: the Hero cost less than a silicone sac.)
A little clear nail polish and some cursing (while trying to line the hood up with the nib) later, the pen fills with ink properly once more. The sac is a little shorter than before, but it still holds a good amount of ink -
So the Parker 21 Super writes again… thanks to a Hero-ic sacrifice!
Very nice. I have a 21 Super that I got off ebay and it was in perfect working order. But now I know one way to go about replacing the sac if it ever springs a leak. It’s not a 51 but there’s something about this pen. Besides, a Parker 51 in Rage Red costs much, much more than a red 21. LOL
Don’t throw the Hero away after slicing off its sac. Give it another lease of life by fitting an “opened” pilot/namiki cartridge over the stump!
My Hero too gave its life to my grandfather’s sheaffers and has been ressurrected.
But… how would you fill the cartridge…? /scratch head
Use a syringe. See youtube. Or u can even use an eye dropper to carefully pour ink into the cartridge as the mouth of the cartridge is quite large.