The first vehicles equipped with hydraulic braking systems circulated in 1924. Alcohols, particularly polyhydric alcohols such as glycerol, and glycerol-water mixtures were used as hydraulic fluids. Actually, these products didn’t match the requirements of today’s technology and are only of historical interest.

There are three main classes of brake fluids.

The main classes used today are glycol-ether based, but mineral oil (Citroën Hydraulic Mineral Liquid LHM) and silicone (DOT 5) based fluids are also available. Silicone oils are mainly used on military vehicles in United States and mineral oils are used for very specific applications (Citroen, Rolls Royce etc.).

 

Composition

Application

Spec

Advantages

Drawbacks

Glycol ethers and derivatives

95% of world market

DOT 3, DOT 4, DOT 5.1

Miscible with water
Low compressibility
Elastomer compatible

Hygroscopic

Mineral Oils

Citroen

LHM

High boiling point

Not miscible with water
Limited rubber
compatibility

Silicone Oils

Antique military

DOT 5

High boiling point

More compressible than glycol fluids
Very expensive
Need specific elastomers

 

The fact that brake fluids are hygroscopic can be both an advantage and a drawback.

AN ADVANTAGE,

because water is everywhere and moisture from air can permeate through microscopic pores in the rubber brake hoses, nylon master cylinder reservoir, and through various seals from the hydraulic system. As brake fluids absorb water into solution, the local concentration levels are typically quite low to avoid corrosion and when exposed to low temperatures, the solution can also prevent the water from freezing on its own.

A DRAWBACK,

because water increases the solution viscosity at low temperature and decreases the boiling point of the solution. Fluid containing moisture heats up and turns to steam inside the system creating vapor pockets. Brake fluid becomes thus more compressible losing its ability to transmit the force necessary to effectively stop the car safely.