Brake fluid performances specifications
DOT 3 brake fluid
DOT 3 brake fluids are usually glycol ether-based, but that is not because they are required to be. In fact, FMVSS116 doesn’t precise the chemical composition of brake fluids. It simply dictates the fluid physical properties. However, brake fluid industry has, by consensus, decreed that glycol ether fluids are the most economical way to meet the requirements.
DOT 4 brake fluids
DOT 4 brake fluids are also glycol ether-based but they contain in addition borate esters in order to improve some properties including increased dry and wet boiling points. DOT 4 brake fluids have a more stable and higher boiling point during the early portion of their life, but ironically once the fluid does actually begin to absorb water its boiling point will typically fall off more rapidly than a typical DOT 3 brake fluid. By FMVSS116 standards, DOT 4 brake fluids must have a minimum dry boiling point of 230°C and a minimum wet boiling point of 155°C.