An apparatus, system, and method are provided for bypassing a coolant reservoir with a fill tool connecting a source of vacuum pressure and a source of coolant (e.g., a vacuum coolant filler) to a vehicular cooling system inlet port. The fill tool bypass allows the coolant reservoir to be manufactured from a relatively structurally weak polymer material, which is subject to failure under vacuum pressure, while still allowing vacuum coolant filling to be used to charge the vehicular cooling system. Without the use of the bypass fill tool, the coolant reservoir would rupture under the vacuum pressures applied during vacuum coolant filling.