Air is not generated its sucked in from somewhere, and you just wrote where its sucking it, the radiator cap flange and the overflow nipple - if it can leak water it can suck in air too, The overflow nipple and the pipe all the way into the bottle dipped into coolant/water should be air tight, its function is to deposit expanded hot water into the tank and suck it back once the engine cools down. If there is a leak there it will suck air instead of water because air is lighter than water. Keep in mind that a leak anywhere in the system will cause air to be sucked into the system when it cools down. Normal working pressure of the system is 1 bar - about 15 psi, the system should be checked at this pressure.
regular epoxy wont work on the radiator tank overflow nipple, and the one that will work is quite dangerous when mixed, it can burn through your skin very easily. SO its best to either replace the top tank of the radiator or get a new radiator.