I'm sorry Georges, I don't understand your statement.

All auto cooling sys I'm familiar with have rad cap on top of radiator. They are designed to have no air in the cooling system. Shortly after you start up the car, coolant flows from cap 14psi valve to overflow bottle. This pressure increase is entirely due to coolant expansion due to heat. When you shut car off, the pressure drop in the radiator draws in coolant from cap 1 psi valve connected to  overflow bottle back into the radiator. Thus the radiator is always full to top.

Al W.

Mercedes, Volkswagon, Volvo (to name a few) have/had radiators with no cap, connected to an expansion tank that you fill about half way.  The expansion tank – higher than the top of the rad -  has the 13-15 psi cap.

FWIW;

Al G.