HELP!

I've located the source of a slow buy annoying brake fluid leak that has recently become faster and alarming. It's below the brake fluid reservoir and above the distribution block, and when I finally I found the leak I noticed a disintegrating white o-ring hanging out of the fitting where the leak seems to be happening. This, I assume, is the culprit.

I've attached a pic to this and the next message.

Most of my brake experience has been at the wheels (done rotors, calipers, pads, etc.) so I'm in sightly unfamiliar territory, though I have a pretty good working knowledge of brakes in general. Any guidance you can offer is most welcome, such as:

- Is this a common failure point, and is a quick o-ring replacement possible?
- Looks like a special type-size of ring--I can't find an reference to it in the 7/9 FAQ or on the board, so any help here is very much welcome.
- Anything special to look out for in tackling this?

Many thanks for your help.