You need this reservoir. That's where the fluid goes when the pressure inside your radiator gets too big and from where it returns when the pressure drops.
If it don't give you tears, you don't have a heart.
YouTube - ‪Top Gear Tributo a Ayrton Senna - Parte.1 (Legenda PT BR)‬‎
YouTube - ‪Top Gear - Ayrton Senna - Part 2‬‎