A piece of tubing at the front of the crankcase underneath the radiator shroud
your 85 K100 may or may not have the crankcase tubing due to the date of manufacture and when those emissions standards enacted

which is connected to a gray check valve via hose somewhere under petrol tank

which is connected to the forward tubing under the tank of the 2 tubings grouped together
the forward tube is the tank air vent
rear tube is the rain drain of the tank cap

remove the check valve and can use an accumulator cup under petrol tank which makes tank removal easier in the future or
connect long hoses to the rain drain and tank vent tubing
route downward along the backside of the right footpeg plate as above indicated
cap off the crankcase tubing however you deem suitable, a screw, a folded over piece of hose zip-tied, a golf tee
http://www.motobrick.com/index.php/topic,3653.msg21985.html#msg21985https://forums.bmwmoa.org/showthread.php?4534-Why-is-the-85-K100-the-quot-Finest-Year-K-quot&s=a6655a3c79c7707c65cc85f6da573ee9