BRAKE FLUID - DOT 3, DOT 4 explained
BRAKE FLUID – WHAT IS DOT 3, DOT 4 etc.. ?
Brake fluids are classified by both “dry boiling point” and “wet boiling point.”
They are also classified by (DOT) rating, DOT 3, DOT 4, DOT 5, and DOT 5.1.
DOT 3 fluids are usually glycol ether based. By definition, DOT 3 fluids must have a minimum dry boiling point (measured with 0 percent water by volume) of 205°C and a minimum wet boiling point (measured with 3.7 percent water by volume) of 140°C.
DOT 4 fluids are also glycol ether based, but have a measure of borate esters added for improved properties including increased dry and wet boiling points. DOT 4 fluids must have a minimum dry boiling point of 230°C and a minimum wet boiling point of 155°C.
The original DOT 5 fluid specification was expected to be fulfilled by silicone based (SSBF) composition. It was designed for use in applications where its resistance to water absorption (and therefore low corrosion) was desired - like in military equipment. It has also found use in antique cars because it does not dissolve paint finishes.
DOT 5.1 - non-silicone based fluids developed that meet DOT 5 wet and dry boiling point specifications and viscosity requirements. DOT 5 & DOT 5.1 fluids must have a minimum dry boiling point of 260°C and a minimum wet boiling point of 180°C