A liquid additive injection pump for injecting a predetermined amount of a secondary fluid into a primary fluid stream wherein a fluid mixing chamber is disposed within the pump housing assembly. A gasket having at least one one-way valve is positioned between the pump assembly components and mixing chamber thereby allowing a primary fluid to pass through the pump assembly and into the mixing chamber where the introduction of a second fluid occurs. The mixed fluids are then discharged through an outlet. The one-way valve gasket assembly acts to prevent the mixed and/or secondary fluid from passing upstream of the gasket assembly and into the pump assembly.