Radiator pressurized air?

Hi,  

I have the car listed above, American specs at 106,000KM currently. Recently I had to do bypass on the heather core since it started leaking and heating is really not needed where I live. However, I have noticed now when the engine is cooled overnight, and I open radiator cap pressurized air with a bit of coolant goes out through reservoir overflow that causes me to need to add a bit of coolant in radiator.   I have changed the thermostat, checked the water pump, replaced the radiator cap as well with a genuine one.  However, the issue is still there.  I thought it could be gasket, but I don't have any sign that could state that except that air by the way it lasts literally a second after I open radiator cap.  Don't have overheating, no white smoke, checked oil, no bubbles in coolant nothing at all to point me out in right direction.  Any ideas?  Or think that is gasket?   Thank you