Got it. Tell you what. Keep a 1.5 ltr water bottle in the car with you just in case. Before you leave the house fill the radiator and close it FOR THE LAST TIME. Once the radiator is filled, and closed, and you've sworn you will never open it again, fill the plastic 'reserve' thingy bottle connected to the radiator to its FULL mark (or a wee bit less). Make sure the air suction pipe, which just terminates in the air, is free and clear of anything that can block it's intake.
Now DRIVE like the wind, and see if the car heats for a day or two. Don't check water until the car shows extraordinary heating. At the end of the day, if there is no heating, check water. . . but ONLY in the plastic bottle.
If it is less, refill to FULL mark.
Case in point. The radiator was never meant to be opened unless there's something broken. Don't keep opening it. The radiator heats the water, which generates pressure, which pushes or pulls water from the plastic bottle through the air intake. This is NORMAL. Don't touch anything unless the car heats.
Hope this helps.