Abstract: This invention relates to a pressure plug for sealing a service valve fitting aperture in a fully pressurized fluid transmission pipeline to enable service valve fittings to be replaced without interrupting the flow of fluid in the transmission pipeline.The plug, which is located below an elongated main shaft, is comprised of spaced apart cap members having a compressible or resilient material there-between and an inner shaft extending longitudinally through the central axis of the plug so that when a nut associated with the inner shaft and plug is tightened it expands the plug making it larger in diameter than the service valve fitting aperture in the pressurized pipeline. The fluid pressure forces it against the inner wall of the pipeline, thus sealing the aperture.