constant cooling topping up when cold

[celrx]

strange issue on my 94fd. when i open the rad cap (when cold) the coolant is at the top, then it starts to go slowly down. so i top up and it repeats over and over. Until if i close the rad cap and start it, it will overflow out the top of the overflow tank then eventually stop. But if i open the rad cap later its at the top then it goes down again. There are no leaks i can see and new rad cap made no difference has aftermarket ast? I've left it as is and it runs ok and coolant buzzer does not sound. any ideas why coolant only goes down when i open the cap?

 

[Sturneyrx7]

Pressure. This is fine.

When the coolant gets hot it expands pushing the coolant into the AST. Once it cools it contracts but doesn't go down until the cap is removed realsing the pressure.