cooling system question

[FFRWRX]

First, I did a very minor mod to my cooling system. I made a spacer plate to move the degas tank a little further from the engine. Yet another brilliant design, put a frame member right over a filler cap. But that's not why I moved it.

Screenshot 2023-08-05 at 6.08.48 PM.jpg

The reason I did it is that the hose that the arrow is pointing at was kinked...badly...maybe even completely closing it. Pretty sure this is all stock Subaru stuff. So moving the tank a little eased the kink in the tube.

Now, the question. My car is just legally on the road now, so I've gone for several short drives. Everything was fine...until it wasn't. The car was overheating quickly and badly. I didn't venture far from home, so not a huge issue. But really annoying. Then I remembered that I turned the heater on for the first time to see if it worked on a previous drive; not much heat out of it but I wasn't concerned. The A/C worked very well so I was pleased with that. So it occurred to me that the heater was filled with air and turning it on added a lot of air to the cooling system. After relocating the degass tank I checked the coolant in it....pretty much none. Filled it up and went for a drive. Of course it still over-heated, but when I got home and let it cool the tank was empty again. Filled it, started the engine and revved it a number of times, tank level very low again. Repeated that a number of times. Then let it warm up completely and turned it off. I'll check things when it cools completely.

Oh, the question (finally they say). The degas tank is supposed to refill itself if needed from the overflow tank. Everything is properly connected as far as I can see. But looking at the overflow tank, it is sealed; I can't see any vent hole in it. So how is the system supposed to suck fluid from that tank when it is sealed? It would have to be vented in order to allow that to happen. And that must be why my degass tank doesn't refill itself when cooled down. Must be vented somehow that I'm not seeing.

Rick

edit: After searching around I'm finding different names for the same components. The metal one I'm calling the degas tank is sometimes called the expansion tank, and even the overflow tank. I think expansion tank is the more common name. And maybe overflow tank for the plastic one?