Anyone know how to fix a sheaffer touchdown? I have a question

I have two touchdown sheaffer pens.

One I bought and another one was my grandfather’s which I found in my grandmas home after she passed.

I bought one just to tinker with before messing with my grandpa’s and also to maybe use donor parts from.

The one I bought I fixed up and works fine but I can’t seem to get my grandpa’s to work.

For both I changed out the O’rings and ink sacks. But one one works?

I did find a crack in the barrel of my grandpa’s so I swapped that out with a new O’ring but when I go to fill up the ink, it’s like the air escapes the top of the pen next to the o’ring, but only when the the plunger is pushed all the way down. If I push it like 75% of the way, no air escapes, but also the pen doesn’t fill up.

I just bought a little container of silicone grease which I’ll put on the o’ring to see if that helps, but am I missing something? I’m just confused because I don’t see any difference between the one I got working, and my grandpa’s.