hello
maybe an axel seal,or number 25 in the top picture,the cover plate.Ill let you know i am not farmiliar with an external brake system and what could leak down on it,from the pictures it doesnt look like the hydro sits near where the breaks would be,but maybe a trunion seal or the seal between the hydro and rear.
For some reason the pictures came out real small but i can get a bigger one if you want me to point out the seals.
jason