There might be a small leak from headgasket pushing gases thru the cooling system. This can be cause thru detenation. As i said i was in a similar situation and learnt the hard way by changing thru 2 head gasket only finding it wasn't the issue it was detenation and cause the head gasket to leak.
The added pressure is there which cause the overfill bottle to fill up, and not return to the system.
Here is a simple test. When cold start up the car, lets say 2-3min. turn off and remove the radiator cap. When you relase pressure cap if abit of coolant comes out thats seems normal, but if a shitload of coolant bursts out then you have a headgasket problem.
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