Even an intact elastic seal will not always be tight. The reason for this is a complicated interplay involving drag flow, the direction of movement for the areas to be sealed, the shape and hardness of the seal and the durability of the medium. The film of fluid on the wall associated with the escape lifts the sealing lip and therefore remains on the surface. When the movement is reversed, this film comes into contact with the harder rear side of the seal and is partly or completely scraped off: the seal is not tight. This process is accentuated for stickier oil and at higher sliding velocities, since the film becomes thicker in such cases (Figure L 12).
Figure L 12: Leak behaviour of an intact seal