Systems and methods for replacing coolant of an x-ray tube assembly having a closed cooling system include a service port that is operatively connected to a portion of the x-ray tube assembly and a vacuum assisted service kit that is operatively coupled to the service port. Used coolant is drained from the x-ray tube assembly, and thereafter a vacuum is drawn on the x-ray tube assembly via the service kit. Replacement coolant within a vacuum tank of the service kit is degassed under a vacuum. The degassed replacement coolant is provided into the cooling system from the vacuum tank, preferably by pushing under pressure with an inert gas to prevent the introduction of any air into the replacement coolant. The replacement coolant may be pressurized in the cooling system with the inert gas. Thereafter, the service kit may be disconnected from the service port.