I recently changed the hydraulic clutch fluid on my Honda ST1300, I have done so every other year with clutch and brakes. This time, the bike had not been ridden for a few months and the clutch wasn't functioning. It felt like it had air in the system and apparently did, since after I changed the fluid, the clutch worked fine. How would air get in this closed system if fluid hadn't leaked out? I've never had to add fluid to either the clutch or brake reservoirs except when changing it, and it's never been low. There was a small amount of particulate in the first little bit of fluid that I pulled from the bleeder, but after that, I only got fluid.

I know nothing about the conditions under which your Honda was stored or last ridden, and neither did you tell me if the motorcycle was started or ridden after it was taken out of storage. So all I can do is offer a semi-educated guess and suggest that most likely, a small amount of moisture in the form of condensation accumulated in the system while the motorcycle was in storage and then was turned to vapor when the engine was started and run.

You may not have detected any air in the system while exchanging the old fluid for new; but even a tiny amount of air in a motorcycle's small-volume hydraulic clutch system can give the lever a spongy feel and cause the clutch to operate lethargically.