Upon changing the oil in the 1985 K100RS' final drive, I learned there has been water intrusion.  I can't for the life of me understand how/when.
As an airhead rider for decades, I'm well aware of the need to avoid high pressure water streams near the final drive vent on the top of the case.  I always managed that without any problem on my airhead BMWs. 
When I first acquired this motorcycle I changed all the fluids (among other things) and found nothing amiss.  Now, about 12K miles later, the contents of the final drive looked like coffee with a healthy shot of milk.
I live and ride in an arid environment.  In fact, I don't recall ever riding that bike in rain.  During the few times I washed it, I was careful with the hose, yet there was water where it never should be. Can anyone offer an idea that would explain this?  Are the vents on K100 final drives defective

There was something about changing the vents on the drives but I cant find anything about it at the moment.
Ingeapedia will know more.

yes ingepadia was one the ball when  a similar problem occurred

from memory there was a change ....and to be certain that the breather hole faced the back from memory ...

I also experienced similar things on one occasion ...and it wasn't when we went through a totally submerged creek crossing ...after an oil change from memory and possibly the breather had rotated around ...and a wash or two .????

The change was late -84, so your -85 should be OK regarding this mod.
...unless your one is from the very early production that model year.

As Charlie says, check that the breather hole facing backwards.............
and that the o-ring in the bottom of the breather is undamaged.

Must check mine but don't see any vent. No water in it either!!

I had a look at the final drive case on my 1985 K100RS.  I'm none the wiser for having done that.  What I assume to be the vent on the top of the case is simply a plastic cap/cylinder affair.  It's not clear which direction it is facing because there is no opening and all the surface is the same. 
It's completely different from the old, air cooled BMWs that I know well.

brickrider wrote:  It's not clear which direction it is facing because there is no opening and all the surface is the same. 

Remove the breather from the FD...pull straight up...or use a couple of
screwdrivers and push from the bottom.

Drill a 3mm hole 6,5mm up from the bottom, put a o-ring of suitable size at the
metal sleeve on the FD to close the bottom of the breather before you push the
breather back home.

The genuine o-ring have part # 33 11 1 454 506.

This is what my breather looks like (the black thing on top of the FD)...facing the wrong way, as pointed out by Inge. The hole should be facing the rear of the bike.

As suggested, I pried the black cap up and off the breather tube on the final drive housing.  The cap had no hole.  It now has a 3mm hole.  There was no o-ring to be found there, so I'm off in search of that.  My first stop will be a shop that sells all sizes; if they cannot help me, then I'll go to BMW, wallet in hand  
Upon examination of the vent, I don't really understand why the o-ring should be needed. If water should enter the small hole in the cap, it could seep out the understand of the plastic cap if no o-ring is present.  However, if the o-ring is present - and a snug fit - that wouldn't be possible and the only exit would be down the vent tube into the FD case. Of course, his scenario assumes the hole in the cap is not oriented to the rear.
But, what do I know about K-bikes?  I'll fit an o-ring, as suggested by the expert(s)!
Thank you for heading me in the right direction.

brickrider wrote:
Upon examination of the vent, I don't really understand why the o-ring should be needed. If water should enter the small hole in the cap, it could seep out the understand of the plastic cap if no o-ring is present.

The o-ring is mounted to prevent the water from entering the bottom of the cap
and being pressed up between the sleeve and the cap and into the FD.

Okay, I understand.
I found an o-ring of the appropriate size, fitted it and now all looks good. I hope this ends my concerns of water mixing with the oil in the FD.
Thank you for your help.
My K100RS is now essentially ready to bed down for the duration of the winter.  During the cold months I'll rely on the wonderful fairing on my R100RS to keep me relatively comfy until Spring, 2015.