NavList:

Michael Josefsson You wrote:

Do you know if one has to break the shellac to remove the upper (or lower?) part of the bubble chamber? I imagine breaking the shellac can make it leak in the future.

Or is there a hidden filling hole somewhere?

No, no, no!  The MkIX is a Rolls Royce compared to an A12.  Leave the bubble chamber alone.  It's its reservoir tube on the side of the chamber you'll need to attack, but only if your sure you need to.  Are you sure you know how to make a bubble.  It needs practice, and once the skill has been learned, you can amaze your friends for years to come. 

You need to look at Bill Morris' 'The Sextant Book' or some of the excellent stuff produced by Paul Brewer over the years.  Both ought to be available online. https://www.ebay.co.uk/sch/i.html?_nkw=mk+ix+bm+bubble+sextant+civilian+maintenance+handbook&mkevt=1&mkcid=1&mkrid=710-53481-19255-0&campid=5338792814&toolid=10001 . 

The tube was nipped and sealed at the top with a blob of solder.  Paul manages to cut the blob off, tap the nipped bit, and put a tiny screw in.  I have to climb into the roof shortly.  I'll try and find one of my botched attempts to reseal with solder to show you.  My excuse; they must have had better solder in the 1940s and 50s.  When I find my phone I'll copy the bubble making page for you.  DaveP