I found a hairline crack about 4 inches long on my radiator with coolant bubbling. Four days ago, I changed the radiator, thermostat and its gasket, and hoses. I've checked fluid levels and every interface and the inside of the gravel/engine shield. I see NO leaks. I am unaware of any white smoke coming from the tail pipe, but I'll look more carefully. However, I noticed that over the days, the overflow tank coolant level is decreasing.

Could this be due to residual air in the cooling system that is leaving and being taken up by coolant, thereby lowering the coolant overflow tank level?

What else could be causing this?

Thanks!