Brake Fluid

Brake fluid is possibly the single most neglected component of the automobile. Most high performance drivers check their tire pressures and change their engine oil at frequent intervals, but virtually no one ever changes the brake fluid in their street car.

The function of brake fluid is to provide a medium to transmit the driver’s foot pressure on the brake pedal through the master cylinder(s) to the callipers in order to achieve braking.

The foot pressure is multiplied by the mechanical pedal ratio and the hydraulic ratio of the master cylinders, booster (if used) and caliper piston(s).

This is a simple concept. When fresh, all brake fluids are virtually incompressible and the system works as well as its mechanical and hydraulic design allows. There can be, however, significant problems in the proper functioning of brake fluid. Overheated brake fluid can (and will) boil in the calliper. Boiling produces gas bubbles within any boiling fluid. Gas is compressible so boiling brake fluid leads to a “soft” brake pedal with long travel. In extreme cases overheated brake fluid necessitates “pumping the brake pedal” in order to get a pedal at all.

Brake fluids are classified by both “dry boiling point” and “wet boiling point.” They are also classified by Department of Transportation USA (DOT) rating, DOT 3 & DOT 4.

The Government specifications for brake fluid, FMVSS 116 DOT 3, do not dictate the chemical composition of a given classification, or “grade” of brake fluid. Instead FMVSS116 defines the properties of the fluid, such as dry and wet boiling points (referred to as the equilibrium reflux boiling points, dry and wet), viscosity of the brake fluid grade at certain temperatures, high temperature stability, corrosion characteristics, and the effects of t

he fluid on seals, as well as other physical properties like the tendency to jell or separate (called stratification) or form sludge and/or crystalline deposits. Boiling point and viscosity are the most relevant properties to most consumers, including high performance customers. Viscosity is an important factor for proper operation of ABS and Active Handling Control systems on modern vehicles since in most cases the pressure and volume of fluid transferred is not measured. Instead, flow through a valve with a given orifice size over time are the control mechanism, so fluid maximum viscosity is a key characteristic