Ok Joe maybe I am wrong but here goes, it takes hydraulic pressure (fluid) to push against something (a rubber seal) that then pushes a cylinder, rod, or whatever, so that fluid that is pushing the seal is sneaking by it and not applying full pressure, it is leaking back into the system (back to the resevoir)not exteranlly but losing the initial pressure that it had gained due to a faulty seal. Call me crazy but this is my thought.