Hi Steve, I’ve attached a sketch of our two systems as I understand them.  Your system is equivalent to a gas can turned upside down with a very small outlet hose and the air cap sealed tight – no, or at least very little flow out the bottom.  I’m pretty sure if you de-pressurize your system, remove the expansion tank cap and come back 2-3 weeks later, the level in the expansion tank will have increased.  You won’t see any change during a quick top-off in a couple minutes.   If you open your Schrader valve it will overflow in a few seconds.  I understand your systems’ equilibrium once it is topped-up and you are in maintenance-mode – but how do you fill your engine when bone-dry? I assume you close/pinch the lower hose and fill the block from the top as complete as possible; at this point you can seal the Schrader valve, add coolant to expansion tank, open lower hose, pressurize it and force it into bottom of engine while slowly opening Schrader valve.

Jeff