I just got a Waterman 452 1/2V from Ebay. It was described as being in "excellent condition". Upon arrival it got a basic cleaning. Pulling the fill lever, it was clear that the sack was either dust, or entirely missing. I just happened to have some number 16 sacs sitting around here (slated for a Lady Patricia restore). And they looked about the right size for this pen. My guess was not entirely correct, but the work-around was easy enough. With a #16 sac, it fit over the section well enough, but coaxing it down into the pen itself proved problematic, as it wanted to fold more than feed. The plate which compresses the sac takes up just enough space in the barrel that it was difficult to get the sac into the tube.
It occurred to me that if I could make the Sac more rigid, inserting it would not be a problem. Some crazy ideas were considered, such as filling the sac with water, letting it freeze to provide rigidity, but that idea was discarded. The most simple approach looked to be removing the Nib and Feed, and then sliding a Q-Tip down the hole to the bottom of the sac, which would provide the rigidity I needed. I had worked with other pens that either required you use special tools to remove the nib and feed, or you needed to unscrew them. I was at a bit of loss to guess if there was any nefarious traps awaiting me in the nib and feed removal process. Moreover, I could find no videos for the specific model of pen, but there were standard 52 videos, which gave a few hints. Postings within this forum gave a few more hints. The darn thing just presses in place, no special tools or indexing required. With a bit of cold water ran through it, and some gentle persuasion the nib came free first, the feed required some gentle tapping (using a Q-Tip as a drift). At this point, the sac which had just been fitted in place, but not yet had been bonded with shellac was affixed in place with shellac.
Now the pen must rest all akimbo throughout the night to allow for curing of the shellac. Tomorrow evening it will get its final assembly, and a bit of tuning (if needed) for the nib/feed. The nib seems to have a generous amount of flex, so I am looking forward to seeing how it writes. Some of those old number 2 nibs make surprisingly good flex pens.
The pictures below are of the advertisement on Ebay, the other picture is of the disassembled pen.
When it is assembled and writing, there will likely be a follow-up.