Especially when replacing the injectors in your 24 valve Cummins, its very important to also replace the connector tubes, located in the head between the injector and the injector line. This tube presses against the injector and passes fuel to the injector, so a perfect seal is necessary to avoid fuel bypassing into the head. In many instances, the original tube wears itself into the original injector, creating a circular depression that does not allow the old tube to properly seat into a new injector.
Connector tubes are sold individually and the o-rings are available separately. We would highly recommend replacing the o-ring with each connector tube.