A coolant filler neck assembly for a pressurized cooling system. The coolant filler neck includes a cylinder housing defining a non-pressurized coolant overflow reservoir; an upper cap member attached to an upper region of the cylinder housing, the upper cap member including an overflow inlet port member arranged internally in the coolant overflow reservoir and a filler pipe member in fluidic communication with the overflow inlet port member and the pressurized cooling system; a first fluid overflow tube member arranged internally in the coolant overflow reservoir to extend in a longitudinal direction though a first region of the non-pressurized coolant overflow reservoir to direct overflow liquid coolant to the non-pressurized coolant overflow reservoir; and a second fluid overflow tube member arranged internally in the coolant overflow reservoir to extend in a longitudinal direction though a second region of the non-pressurized coolant overflow reservoir to direct the overflow liquid coolant from the non-pressurized coolant overflow reservoir to the ambient environment.