LOLZ @ " It's just a bottle "
Mr. Xulfiqar, maybe u hav'nt experienced such in ur riding history.
By using generic same volume 2nd copy OR 3rd company made(non-genuine) are all useless. Besides they keep cracking! due to poor plastic quality.
They may work in salon cars OR other vehicles who don't have proper functionality flow-chart.
As the engine temperature increases the coolant expands, plus getting very very HOT, so hot that its impossible to keep ur finger for 2 seconds max. As the coolant expands & gets HOT the volume in radiator is unable to hold it, hence releasing it through pressure controlled cap hence throwing the HOT coolant/water into the Reservoir.
As the engine cools down again it sucks in water/coolant back again thru the vaccum created in the system.
If the reservoir bottle leaked/cracked or even its cap is disturbed or not tight, its looses it vaccum capability & suck air hence loosing the pressure hence P=F/A, i.e Pressure = Force/Area.
This system helps by not loosing coolant/water from the radiator & removing the tension of re-filling it again & again. This system is reliable/simple & works perfectly if assembled properly with good parts!