oh dear I didnt know this was going to be so complicated. Riff Raff although you dont fill your rad completely or it burps I bet that every time you remove the rad cap after having used the bike a few times its full of coolant. This is because as the liquid expands on heating it pushes the air out to the expansion bottle where its lost then on cooling coolant is sucked back into the rad to replace all the space available.
Also you shouldnt top up the expansion bottle while hot because its level isnt stable but goes up and down depending on the engine temperature. Once cool the level can be correctly assessed although even then it varies a little with ambient temperature........
Next one: cars have both types of system and if you fill (not just top up) your cooling system through the expansion bottle its the later one (not SV). Heres an example on ebay of this type of expansion tank with pressure cap:
https://www.ebay.co.uk/itm/VAUXHALL-...UAAOSwJo5bFfJL
In both designs if overheating occurs (for example due to fan failure) the pressure in the system will go up and has to be released by a pressure cap either on the rad or on the expansion tank depending on which type you have. The SV type will discharge the excess pressure from the rad to the expansion bottle where it should cool and not produce much steam. The other type however discharges from the bottle usually by a hose down under the vehicle and can produce steam.