Abstract: A removable manifold assembly for attachment to the outlet end of a multi-cartridge injection device. The manifold assembly comprises two rigid plastic housings which encapsulate a rubber septum. Check valves are formed by compressing the front surface of the septum against a seal face of the rear housing to form a fluid tight interface therebetween. A first fluid path is formed from each cartridge in the injector through the manifold assembly to a patient delivery needle. A second fluid path is formed in the manifold assembly from the first fluid path to a back surface of the septum in order to enable a residual fluid force to be exerted against the back surface of the septum, thereby forcing the respective check valve in its closed position. The manifold assembly includes a unique geometry that permits the manifold to be snapped into engagement with the housing only when properly oriented with the housing.