Bad headgasket, Why is my coolant not returning to the engine?

Hello,

I know I have a bad head. Anyway the coolant overflows out the overflow tank and fails to return to the engine. Figured that CO2 was displacing the coolant and the vacuum created when cooling then was minimal. BUT, when i removed the coolant overflow lid and took the little hose in it and exposed it to air THEN removed the rad cap. My lower and upper hose collapsed/sucked in. What does this indicated?
Added (1). BTW it is confirmed with a block check chemical test. 2000 Honda civic Dx. Aluminum engine. Car overheats only intermittenly after long drives and coolant only needs to refilled because it is overflowing. Also my thermo is working as well as fans.

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