In a plug-style valve, such as an air-conditioning service plug-style valve, basically including a valve body, a stem, a retainer including a service fitting and a valve core, and a cap, the plug including an intermediate O-ring, between the valve body and the plug, disposed in an intermediate O-ring seal groove of a serpentine shape and having integral first and second O-ring groove portions, the intermediate O-ring being stretched a predetermined amount during installation and defining, therebetween and the valve body and the plug, a closed volume without a primary lateral passage in the plug, with the O-ring stretching preferably being in excess of about 15% to offset refrigerant medium absorption effects that will cause the O-ring to swell, become too large and become loose within its retaining groove, with the single cap including a redundant seal that covers both the valve actuation stem and the service fitting