Abstract: A mechanical seal assembly is provided for inhibiting a fluid located in a higher pressure region of an apparatus from flowing to a relatively lower pressure region. The seal assembly is configured for cooperation with a seal ring connected to a cylindrical shaft which rotates about its longitudinal axis and which is mounted in the apparatus housing. The seal assembly includes an outer casing in which is mounted a non-rotatable seal ring which encircles the shaft for limited axial movement toward and away from the shaft seal ring. A biassing element is mounted in the outer casing for urging the non-rotatable seal ring axially into slidably mating engagement with the sealing face of the rotatable shaft seal ring. The biassing element includes a cylindrical bellows which encircles the shaft and a spring. The bellows includes a collapsible outer wall with a first skirt on one end of the outer wall and a second skirt on the other end of the outer wall.