non - flammable hydraulic fluids which are particularly useful for replacing existing hydraulic fluids in hydraulic systems employing acrylonitrile - butadiene rubber and fluoroelastomer seals are disclosed . the non - flammable hydraulic fluids of the present invention comprise a fluorinated chlorotrifluoroethylene oil , a refined naphthenic oil , a phenolic antioxidant , a sulfur corrosion inhibitor , a metal deactivator , and an aliphatic ester . the fluorinated chlorotrifluoroethylene oil is present in an amount of at least about 75 % by volume of the hydraulic fluid . in addition to being non - flammable , such hydraulic fluids are compatible with both the seals and brass components which contact the hydraulic fluid in hydraulic systems .