With no coolant recovery, the coolant heats up and expands and pushes some coolant out of the overflow line. When it cools down, it contracts and air is drawn back through the radiator cap. That is why you often see the fluid level an inch or so below the neck of the radiator. The radiator cap is actually a 2 way valve. With the recovery can, coolant is siphoned back into the radiator to keep it full. Air helps break down the coolan and the corrosion inhibitors. I believe recovery tanks are a big reason new car radiators seem to last much longer without scaling up.
But the odd thing with it keeping pressure is that the cooling system starts out at 0 psig when it is filled. So as it expands and contracts it should return back to 0 psig.
I suppose if the system is filled with cold water during the cold months, it will have pressure on it once the ambient temps heat up.
I can't think of a reason to have any pressure on the system on a car that has sat for days. Typically I will hear it pull in some more air as I remove the cap.
Doug
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