I dont think you understand the point here - your engine has a CR of 22:1, even a slight amount of incomressible liquid can bend the connecting rods and wreck the engine.
And if the leak is so severe that the mechanic could just push some air from the overflow tube and it could push out liquid from there - the engine would be beyond scrap to make that happen.
Check for the little coolant pipes on the pump for leaking.