The coolant recovery system requires that the antifreeze
side of the system must be airtight. As the coolant in the engine heats it will expand. The expanding antifreeze
mixture raises the pressure enough that the pressure valve in the cap pops off and allows the antifreeze mixture to flow into the coolant recovery tank. As the engine cools after shutdown the antifreeze mixture in the engine shrinks. Here's the tricky bit - if the engine is not completely airtight the shrinking antifreeze mixture draws air back into the engine instead of drawing the coolant from the recovery tank. The air leak might be so small that you can't see it. A loose clamp, a tiny crack in a hose or a failing seal on the water pump
shaft can be repaired pretty easily after you finally discover it. If the air leak is a gasket
on the cooling
or a failed head gasket
it will be harder to find. But you must find and fix the leak in order to restore the system to full function.
If the leak is the water pump
shaft you will see a trace of antifreeze at the 'weep hole'. Usually you can find THAT on the pump
casting behind the pulley, (if belt-driven), or at the drive end if shaft driven.
If the system is not air tight what will happen is that the pressure from the expanding coolant will force the valve of the cap open as well as forcing a tiny bit of coolant out of the leak, so find the leak and fix it. Once the system is airtight again the coolant will go back into the engine after shutdown.