Problem Re-Saccing Waltham Button-Filler

[Sailor Kenshin]

DH and I recently began fiddling with old beater fountain pens. We are not after full restorations at this point, just writers with character, and have managed to re-sac seven lever-fillers, three of which have become excellent writers (and some of which need further work). Now we've moved to button-fillers, and we ran into a problem. I'm quoting him here:

 

 

'Notes & Questions:

 

When reassembling this pen, if there is no sac fitted on the section, the section fits in the body properly (a pressure fit) and the button and pressure bar seem to work normally. However, once a sac is attached to the sections nipple, the section wont slide all the way into the body and slowly creeps out. There is a spongy feel when pressing the section in. Ive tried smaller and shorter sacs with no difference. Its the same problem even if the pressure bar is not installed in the body.

 

Ive noticed a ring or ridge of gunk just inside the barrel at about the same depth of the sac nipple step on the section. Could this just be old sack and adhesive that should be scraped away? Its also where the end of the pressure bar wants to rest.

 

Should I just give the barrel a good cleaning and scrape that ridge out or do I need a special, thin-walled sac for this pen?'

 

Here is the pen, fully disassembled:

 

http://extras.ourpatioparty.com/files/2014/8863/8804/Waltham_BF-fully_apart-640p.jpg

 

 

This is looking into the barrel with the pressure bar in place:

 

http://extras.ourpatioparty.com/files/2914/8863/8802/Waltham_BF-barrel_w_bar.JPG

 

 

This is a tilted view of the barrel showing some of the ridge:

 

http://extras.ourpatioparty.com/files/8214/8863/8800/Waltham_BF-barrel_tilted-640p.jpg

 

Can anyone tell us what to do next? Thanks!