A sealing device which is used as part of an injector in a reductant delivery unit (RDU), where the RDU is part of a selective catalytic reduction (SCR) system for injecting diesel exhaust fluid (DEF) into an exhaust system, used for controlling exhaust emissions. This invention improves the purging efficiency of the RDU, in addition to improving the sealing performance of the unit by reducing the fluid volume to be evacuated from the injector, and provides an additional sealing barrier to reduce the sealing load of the existing sealing elements. An additional sealing device having a ring seal and a stopper element (also referred to as a “plug”) is pressed into the upper valve body of the injector, which results in a reduced fluid volume that is left behind after a purging event, due to the reduced initial volume at the beginning of the purge.