oil leakage from bevel joint

[mel holmes]

I have a R100GS 91 paralever and every now and then get oil leakage from the bevel joint oil breather, sometimes even making it's way onto the back wheel - I first noticed it when the back end slipped! It only seems to do it on long hauls and oddly enough the oil level does not seem to be going down. I thought maybe oil was making it's way out of the gearbox and down the shaft - I did find traces of oil there but the gear oil doesn't seem to be going down either! Could that get into the bevel drive anyway? My brother has the same bike, same year and has exactly the same problem.

Cheers

Mel