[SV650rules]

Modern car and bike systems are identical pressurised systems with expansion bottle, they are doing the same job with the same coolant so why would they be different ? I know my car has a pipe from radiator filler cap neck to the expansion bottle and the rad cap seals on top of the neck, so only way for fluid to get in and out is to and from expansion bottle via the pressure valve in the cap, exactly the same as my AL7, and unless the rad is filled right to the top neither system will work as designed because any air in the system will expand much more than the coolant when it gets hot and blow air into expansion bottle, when the system cools it will suck coolant back on (the coolant entry / exit to expansion bottle is the lowest point in the bottle so it is always covered by coolant, so system is designed to stop air being sucked back into rad.

The expansion bottle on any car i have ever had has not been a 'pressure vessel' just a plastic bottle with a cap and a tube long enough to reach bottom of bottle, it does not care if cap is screwed on or not as there is a small air hole in the cap to stop pressure build up or vacuum in the bottle. Nothing should ever get dumped to atmosphere unless there is a catastrophic failure (or if you read the warning by the cap 'never remove cap when engine is hot' because the rad will be under pressure and removing that pressure suddenly by removing the cap can cause boiling and spray you with hot coolant) - but the expansion tank is not pressurised.

ideally the pipe from rad to expansion tank should be full of coolant with no air in the pipe, this means that the only thing to be sucked back into rad would be coolant not air, but every time you take rad cap off the coolant drains back into expansion tank because it is lower than neck of radiator, but then again if expansion bottle was higher than neck of rad the bottle would empty itself every time you took rad cap off.