Recently completed a coolant flush on my "new to me" '92 ST1100 (PO had no idea when completed last) and this forum was invaluable with all the steps, tips, and tricks discussed. Thank you all!
+1 on why that reservoir tank in back needs to be pulled, emptied, and flushed. Upon doing said coolant flush, I found that tank full of thick slime rather than liquid.

With everything disassembled, I also replaced the 5 ft of reservoir tube, as well as the radiator cap and thermostat.

The system is now nicely sealed and engine temps are steady. Even in stop/go Phx traffic the gauge really doesn't push past the 'm'.
I know it's posted somewhere else, but for convenience will list again the 3 key parts I was able to score from local auto parts store for this project.
Radiator Cap = '94 Accord 2.2 engine
Thermostat = '80 Civic 1.5 engine
Thermostat o-ring = '80 Civic 1.5 engine
Getting those part from local auto store saved obvious $ and time! Again, big thank you to the forum community!