If the hoses are soft, then it obviously not pressure you hear when releasing the cap.
You said that you either have:
1. No coke sound and coolant level normal, or
2. Coke sound and high coolant level.
I submit that the sound you are hearing is a vacuum because it has failed to draw coolant from tank (high level)
when engine cooled.
I am confused now, if it's vacuum then the system is functioning properly, but why it doesn't pull the coolant from the overflow back then? Return hose is clear of obstructions because if I blow into the overflow bottle i can push coolant back to the radiator (found it easier that removing tank to refill it).
If the coolant level in the recovery tank is at the hot level when the engine is cold, then:
1. Coolant cannot be pulled from tank because of a blockage somewhere in that path and there would be a vacuum in the system, or
2. There is a leak somewhere else in the system and when the engine cools, it pulls air in instead of coolant from the tank. If the radiator neck is deformed (or cap) so that the large gasket on the cap will not seal against neck, then the engine will pull air there instead of from tank. That is just one example.
Ok, I have checked radiator neck for deformations ( used piece of glass, flattest thing I had) and everything is ok. I noticed something yesterday, bu at this point I don't know if it's really symptom of something or I am just being paranoid and seeing stuff that isn't there, but I kinda feel that my engine is not working as smooth as before while idling. You know when you have something for a long time ( i had this bike from new , 200 and notice if something is off, I have a feeling that it just idles rougher, but just barely noticeable. And also, one of the exhaust pipes (upper, from rear cyl) sounds like it , I don't know better description, pops here and there. It is really hard to describe, I tried to capture it on video but of course, it will not do it then, and I don't trust myself ATM, but maybe it is something that could steer you in right direction?
Debaffled VN900 - annoying my neighbours since 2008
1. The plugs seem to look not that bad, BUT I would replace them. There have been reports of NGK plugs failing, particularly in 900's for some reason, giving the symptoms you describe.
2. I would do a cooling system pressure check.