When I shut off the bike the radiator hose running from the bottom of the thermostat housing to the water pump collapses like it has a vacuum. Not all the time, but often. Any ideas?

I had the same problem on an 89 K100RS, I replaced the hose but problem still occurred, I believe it is caused by a faulty radiator filler cap but I am sure someone will jump in very quickly with the correct solution.

When the system cools off, the radiator should pull coolant back into it from the black reservoir on the right side of the battery. Check the hose from the filler neck area back to the coolant reservoir to see that it's not "pinched off" somewhere. Additionally, remove, empty & inspect the reservoir to see that there isn't some kind of debris in the bottom that could obstruct the flow of coolant back into the hose. It's possible though unlikely that there could be a buildup of "gel" like material in the bottom from some use of "High silicate coolant" at some time. If all that checks out there is probably some foreign material or other problem blocking the pressure relief valve in the filler neck cap under the tank. I assume the cap has some kind of spring loaded one way pressure valve built into it somewhere, although I have yet to closely inspect one. It should hold some restriction to flow from the radiator towards the coolant tank to allow the radiator to have pressure when hot. As the system cools the pressure relief valve(s?) should allow free flow back into the radiator from the reservoir once the pressure in the radiator returns to atmospheric or slightly below.
I have a used cap that I expect is OK that I could send you for $1.00 + $5.15 for a usps flat rate box should you want it. I have paypal.
Please post back what your inspection reveals.
Regards, ibj...

That is usually the filler cap that causes the problem but check for the others as ibj has said.

I cleaned the relief valve in the fill cap along with the ports that allow the fluid to return to the reservoir. The ports were full of the silicone substance. Ran the bike a while and after shutting down the hose didn't collapse. I'll take it for a ride tomorrow to see.

You might consider changing & flushing (not a chemical flush) all the coolant in the engine and installing a new brand name coolant that is labeled "low silicate". to prevent more silicate "drop out" in the future.
I think probably that all good brands these days are of the low silicate variety, but I couldn't say for sure. Additionally, if you don't buy the stuff that is "pre-mixed" already to 50/50 concentrate then I would use distilled water to mix with the concentrate, not tap water.

Rode about 50 miles today an then let the bike sit to cool with no issues. Looks like cleaning up the pressure switch and vent ports in the cap took care of it. Thanks for the replies.