Run your car normally till gets hot and the radiator spins,turn the heat to 4th scale and keep if for a while Sto and raise the hood while the engine is running at idle Wear gloves and take a cloth-towel,squeeze gently any coolant house you can reach especially the main,this will push the air bubbles to go to the top Cover the cap from the coolant expansion tank with the cloth-towel,unscrew it extremely softly,if a ''hisss" sound heard means you had air bubbles,continue and loosen one click more,maybe the ''hisss" sound will be heard again maybe not,one click more and the cap it's easily loosen till comes out By the way check the inside of the expansion tank for any detritus Turn of the heat to 0 and turn off the engine,screw the cap back but not with extremely force Return home and leave the car to cool off completely at night,next morning check the cap because one click tighten maybe needed (maybe not). You can check for leaks in a repair-shop,they have at least 2 ways to raise the pressure in the system and if there is a problem then coolant drops will show-up in the problematic spot.