Abstract: An apparatus and method for repairing a leak in a vehicle heater core without requiring its removal from its vehicle mounted position. The apparatus comprises an airtight container having an immersible pump and heater element secured within the container and including inlet and outlet fluid lines for coupling with hose extensions from the heater core of a vehicle. Access to such hose lines may be accomplished by an intermediate cut on each of the inlet and outlet hoses to the heater core, joining the respective flowlines of the container into a closed fluid circuit. In this configuration, flushing solution, cleaning solution and sealing compound can be pumped through the heater core to repair any existing leaks. An inlet valve is provided for introducing air pressure to assist in delivery of the sealer compound through the closed circuit and confirming repair of any existing leaks.