The term 'hydro' in the renowned Cooper "S" suspension system "Hydro-lastic" means at least in part a liquid. This famous system was designed by BMC make for smooth and fluid ride while taking all of the abuse of off road rallying. It is not uncommon for some of us to say that it is the best ridding Mini ever. Unlike the more common rubber cones that need periodic replacement, the only thing needed to keep a 'hydro' car up to scratch is "Hydro-Fluid". The offical proper fluid is made by Morris and comes in a one gallon container, more than enough for either a complete re-do or routine maintenance.