The overflow cap on the RADIATOR is pressurized (spring-loaded valve), this bleeds off excess coolant through the overflow tube to the reservoir when pressure gets too high (it releases at 12-14 PSI on most cars). It will not selectively let air out, unless the air bubble just happens to be sitting next to the pressure cap when pressure gets high. This is unlikely, since the air bubble will circulate with the coolant as long as the water pump is running. The bubble will rise to the highest point when the car is off, which is where the coolant burp valve should be located.