Hydraulic seals are essentially o-rings. However, they feature a groove within the lip that allows them to fit the components with which they are used. The groove will slide over the component to create a leak-proof seal. Even when the hydraulic cylinder’s piston moves and its fluid pressurizes, the component won’t leak. The hydraulic seal will keep the fluid within the appropriate container so that it doesn’t leak around the component. http://www.kintoweseals.com/hydraulic-seals/