My new (to me) 85 K100RT (with an RS fairing) has spring a coolant leak. Pressure test shows a gusher making puddles on top of the motor behind the fuel rail. I suspect either a split radiator hose or a leak in the radiator itself. Whichever, it's in front, in the middle. 

Which way should I approach this? Should I be trying to disconnect the hoses and go in from the top? or from the front, remove the radiator and go from the underneath? What did it look like when this has happened to others? Anything I should know? 

Thanks, cheers

Firstly, I would remove the airbox, air filter and airbox intake duct as this will provide unrestricted access into the are behind the radiator.  If it is the hose running into the engine behind the radiator, you will need to do this anyway.  

From there you can easily see any leaks in that area including the rear of the radiator.  Whilst there check the radiator fan rotates freely otherwise the motor is stuffed and/or a Spall Fan can be fitted.  Check the Z tube for cracks as well.  

By removing the above components you are making everything a lot easier and you will be able to collect any fluid to remove it.  Give the top of engine a clean as well.  

The airbox intake duct is a press fit and a small blade screwdriver may be required.  Then undo the three clips holding the top of the airbox in place.  They bend out with a screwdriver.  There is a large worm drive clip at the front which needs to be undone.  Use a ratchet with the correct socket in 1/4" drive, not a screwdriver.  Take note of the position of the drive for refitting.  Remove the air filter which should slide out towards you.  The lip on the edge may catch.  The top box part has the AFM inside connected with a cable, so just tie it up and let it hang outside.  Then remove the two (?) bolts that hold the lower part of the airbox in place.  

Take photos as you go.  You'll thank me later.    

This is how I would do it to check for leaks in there.

One ease-of-use addendum; the AFM connection (not connector!) is a PITA. On all of my K100s I've cut the cable and fitted a four-way connector block. If you ever need to go in there again (you will!) it means you can fully detach the AFM which makes working in that area a whole lot easier. There is a seven-way (yes) connector inside the AFM but it's near-impossible to detach without removing the AFM itself from within the housing.

My guess is that the rad has corroded through next to the semi-circular bracket that locates it on the frame. It seems to be a common failure point - personally I've had two fail there.

It is possible that the fan blades on the cooling fan have slid forward on the motor shaft far enough to hit the radiator.  This results in the blades wearing holes in the coolant passages.  There have been several instances of this problem. 

A hard to spin fan that makes rubbing noises on the radiator is an indicator that this is the problem.

This is all fantastic info, thank you all so much! I will get pictures as I go in. 

Pretty sure this is a radiator leak, from the way it went, and how little pressure there is now. I've ordered a new three-row aluminum radiator.