I just had that replaced on my old 06 R1200GS. I didn't have the tools to do it nor did the dealer. Plus the dealer didn't even want to touch it. Found a Independant BMW bike shop and had them replace it for around $250 including parts. but I took the FD off myself and brought it to him.
I replaced numbers 2,3,5 and 6 in the diagram. When I looked at the old parts. looked like the crush washer got a little dirt on it and killed it making it leak. and get past the seal where the dirt came in..
But if you don't know what your doing. I'm told i wouldn't mess with it and let someone else who does know deal with it..