Brake fluid types
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.
DOT 5.1 brake fluids
Historically, DOT 5-level performance (specifically boiling points and viscosity) could only be achieved with silicone-based fluids. However, modern compositions have created glycol ether-based fluids which now meet DOT 5 brake fluids requirements in these key areas. Consequently, the DOT 5.1 brake fluids level was created to differentiate these two very different chemistries which both meet DOT 5 brake fluids performance requirements.
In so many words, DOT 5.1 brake fluids are simply DOT 4-type brake fluids which meet DOT 5 brake fluids performance requirements. Because of this, they typically can be mixed with DOT 3 or DOT 4 brake fluids without concern. Sometimes, they are even referred to as ‘DOT 4 Plus’ or ‘Super DOT 4’ brake fluids because they are more similar to a conventional DOT 4 fluid by chemistry than they are to a conventional DOT 5 brake fluid. In fact, DOT 5.1 is essentially comprised of borate esters.
DOT 5 brake fluids
DOT 5 brake fluids have been developed for military applications, i.e. for vehicles that could stand in storage for years, without maintenance and had to perform immediately when required. They are superior in terms of boiling point retention and corrosion/conservation properties, since they don’t absorb water. Up to now car manufacturers have not moved to use silicone fluids for first fill regular cars for two main reasons:
- Low air solubility, which leads to a spongy brake pedal feel.
- No water solubility, moisture which enters in the system can be corrosive and can freeze at low temperatures or boil at high temperatures.