The radiator is made of metal and is susceptible to rust. Modern radiator designs incorporate a plastic
top – the area most prone to rusting – but internal parts are consistently exposed to moisture as well.
To check for rust, pop the bonnet when your engine is cold and remove the radiator cap. Look at the coolant level
A brownish fluid would mean excessive rust inside the cooling system and immediate flushing is recommended. Inspect the radiator cap and surrounding area for any signs of rust. Have the radiator inspected if excess rust is present.