Well a quick google search I came across this and I skimmed thru it and saw mentioning of a 4.5mm allen key.
But you really should get a set of allen keys just in case...
http://www.thinkythings.org/p5/brakes.html
... your fingers will thank you for using a ratchet instead of that thin pointy little piece of steel
i don't pay attention i just start grabbing sockets until i find the one that fits lol. but i would recommend picking up a set of the socket style over the regular allen wrenches. you may not need it for this, but when you have to crack something loose that's torqued on there pretty good your fingers will thank you for using a ratchet instead of that thin pointy little piece of steel
Here are some pics of the access bolt for the adjustment gear. 14mm
Bad pics but you get the idea.
...Going to really go through some things tomorrow. I have to figure this out.