Incidentally, the reason oil is behind the primary seal is not, i believe, the reason given by antar in my attached link. I only attached the link because the diagram was the clearest I could find on tinternet.
Antar says oil is behind the primary seal to stop the bore from rusting, but brake fluid is highly hygroscopic, so it's difficult to keep water OUT of it! The actual reason there are two seals is explained in "sportbike performance handbook" by Kevin Cameron. It is to do with stopping fluid escaping from the reservoir once the brake lever is applied. That's also the reason there are two holes into the reservoir and not just one.
The one very important thing missing from a lot of these diagrams is the fact that the top side of the bellows has to be vented to atmosphere. Meaning that the entire system is exposed to atmospheric pressure, but not to atmosphere itself, when the lever is relaxed.
Cheers.
