A fluid valve includes a valve body and a plug. The body includes a chamber and ports that communicate with the chamber. The plug rotates within the chamber, and includes a seal disposed in an annular groove provided on the plug surface. An elastic member is disposed in the groove, and biases the seal against an inner surface of the chamber. When the plug is oriented relative to the body so that the groove is aligned with a port, the seal segregates the chamber into a first chamber that borders an outer surface of the seal and a second chamber that borders an inner surface of the seal, and provides a fluid-tight seal between the first chamber and the second chamber. In addition, the seal inner and outer diameters vary based on a fluid pressure difference across the seal.