The mantra used to be "A master document has only two possible states: Corrupt, or just about to be corrupt. And that is why we say that the only possible fix to a master document is “'don't use it!'”
John McGhie, MVP, on the Microsoft Word MVP FAQ Site
There's a less pessimistic assessment (plus a lot of tips) at
http://techwhirl.com/ms-word-master-documents/ but this still says there should not be any content in a master document. When you say "edits made to the master" and mention updating fields, I'm not sure what your approach has been.
Good luck, anyway, and I'm curious to see what the better-informed have to say on this topic.
[ps] a few hours later, I've had a coffee and read the excerpt from the URL I gave: scary stuff, and makes me heartily glad I don't have to use Word for long documents =:-}