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Brake Fluids

Brake fluids are used for transferring the power from the main braking system to the brake disc. In this case, the function of the brake fluid is similar to that of a hydraulic fluid. In contrast to the conventional hydraulic oils, the commercial brake fluid comprises mainly glycols and some additives, except some (some mineral oils with Citroen, silicon oils with old timers and the army). Therefore, it is also slightly poisonous and attacks the skin and mucosa. Furthermore, glycols are poisonous to fish.

Chemical Properties of the Brake Fluids

Brake fluid mainly comprises glycol (divalent alcohol) and glycol ether with additives, for example for the lubrication capability and corrosion protection. There a primarily three material groups currently that are used as brake fluids.

The chemical composition of the three fluids is completely different; therefore they cannot be mixed with each other.

  • Polyglycol ether base
  • Mineral oil base
  • Silicon base

Polyglycol ether

These fluids are the most widely used and have been manufactured for over fifty years.

Brake fluids with a glycol base are hygroscopic – they take water from their surroundings.

Often, water absorption characteristics are considered to be a disadvantage of the fluids; this behaviour of the fluid is important to determine so that there are no drops of water in the brake system. The absorbed water is dissolved completely and therefore prevents the build-up of drops. Free water drops lead to local corrosion and would freeze in low temperatures (blocking some important holes!).

Further development of brake fluids is working towards reducing the influence of the absorbed water at boiling temperature. In addition, brake fluids are enriched with anti-corrosive additives to prevent damage of corrosion to the braking system through water. This development can lead to an increase in the time between maintenance works.

Keywords: Polyether

Silicon Brake Fluid

Silicon (polysiloxane) based fluids are not hygroscopic. There is no corrosion without water either.

The US army only uses silicon brake fluids. They are also used for various old models.

In Europe, the fluid has not caught on, since the braking systems will need to be designed especially for using silicon fluid. Silicon cannot dissolve water, therefore there needs to be an expensive modification in the design to prevent water from seeping in.

Silicon brake fluid is also more than 20-times more compressible than polyglycol ether brake fluid, which makes the use of larger main brake cylinders necessary.

Mineral Oil Brake Fluid

Mineral oil or silicon oil based fluids are very special high premium oils that have nothing to do with regular motor oil.

It is imperative to ensure while using mineral oils that the correct mineral or synthetic oils are used. The central hydraulics and ABS cannot be used with just any oil! The use of incorrect oil can lead to a breakdown of the entire braking system.

Laboratory Chemical Analysis of Brake Fluids

In our laboratory, the following parameters can be measured through DOT:

Test Method DOT 3 DOT 4 DOT4
Type 200		 DOT 5
Chemical property Glycol Glycol Glycol Silicon
Boiling point

Equilibrium Reflux Boiling Point ºC (ERBP)

 ≥ 205 ≥230 ≥ 280 ≥ 260
Wet boiling point (water content: 3–4%)

Wet Equilibrium Reflux Boiling Point ºC

 ≥ 140 ≥155 ≥ 200 ≥ 180
Viscosity -40ºC (mm²/s) ≤1500 ≤1800 ≤900
Viscosity 100ºC (mm²/s) ≥1.5 ≥1.5 ≥1.5
Density @ 20ºC
pH Value 7–11.5 7–11.5 7–11.5

Other Measurement Parameters

Sources, Links

SAE Handbook

The exact specification of the DOT standard is in the SAE Handbooks.

  • SAE Handbook (Revision 04), complete version: CHF 1,480.00 (Volume 3 has been withdrawn. Only the complete handbook is deliverable.)

It can be bought from the Swiss Standards Association (Schweizerische Normenvereinigung).

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