Brake Fluids

Brake fluids are distinguished from each other using a DOT (American Department of Transportation) number.  The number refers to a set of minimum specifications which the fluid must meet and the two main criteria are wet and dry boiling points. 

Four types are currently available: [Glycol based brake fluids are classed as synthetic] 

DOT 3 – Normal glycol based brake fluid. 

DOT 4 –  Also glycol based.  Chemically compatible with DOT 3, but has a slightly higher boiling point.

DOT 5 – Silicone based fluid.  NOT compatible with DOT 3, DOT 4 or DOT 5.1.

DOT 5.1 – Glycol based like DOT 3 and 4.  Has a much higher wet boiling point.

LHM – mineral based, used in some Rolls Royce systems, many Citroens.  Not compatible with any of the above.

Properties of Brake Fluid

When brake fluids are fresh they are virtually incompressible

Glycol based brake fluids are hygroscopic (that is the correct expression – meaning they absorb water, but often written incorrectly as hydroscopic). This water absorption occurs whether the car is driven for 30 000 miles or sits in the garage.  Fluid contamination is a function of time and humidity rather than mileage. Moisture enters the system past seals and through microscopic pores in flexible brake hoses, it also gets in every time the top is taken off the brake fluid reservoir.

After only a year  DOT 3 fluid may contain as much as two per cent water and after several years service without being changed the fluid is likely to have soaked up around seven or eight per cent water. Most manufacturers recommend changing the fluid every two years. 

This ‘soaked up’ business is important. The brake fluid draws in the moisture and is absorbed into the fluid. As the fluid absorbs the moisture it thickens and its ability to withstand heat and corrosion is reduced. The result is a significant drop in the temperature at which the fluid boils so after a period of heavy braking, the fluid could boil in the calipers. Hydraulics work on the principle that you cannot compress a liquid so the pressure created is used to force the pads against the discs or the shoes against the drums. However, if the water in the fluid boils you can compress a vapour (the steam produced) and this will greatly increase the distance the pedal will have to travel to apply the brakes. 

This isn’t brake fade which happens when the brake linings get too hot after prolonged braking (a descent from the top of a very steep hill/mountain).  Brake fade requires greater and greater pedal effort to stop the vehicle while fluid boil increases pedal travel and makes the brake pedal feel soft. 

As we mentioned at the beginning of this article, the various DOT ratings have different minimum wet and dry boiling points, the higher the number the better the quality and the higher the boiling point and consequently the cost is higher too:

Wet figures indicate the drop in performance when the fluid has a certain water content (less than 3%).  When DOT 5.1 is contaminated with 3% water (which is considered quite a high level) it performs almost as well as DOT 3 in new condition. 

All the glycol based brake fluids are mixable, but if you use DOT 3 in a car designed for DOT 4 you will lose the advantages of the higher specification fluid. 

You can’t assume, either, that early cars used DOT 3 and later ones, DOT 4. Toyota still favour DOT 3.  Many Mercedes cars and vans use 5.1.

There is now another complication in as much as several manufacturers have introduced DOT 4 ESP. This has a much lower viscosity than standard DOT 4 so it can provide improved braking and faster anti-skid reaction at low temperatures.  It is however, compatible with standard DOT 3 and 4.

DOT 5 is silicone brake fluid, purple in colour and not compatible with any glycol based fluid – it doesn’t absorb water, it doesn’t damage paintwork and has a higher boiling point.  However, the fact that water isn’t absorbed by the fluid doesn’t mean it isn’t getting into the system, so rather than the water being held in the fluid, it sits outside it. The danger here is that the water falls to the bottom of the system – usually the caliper – where operating temperatures are very high.  Silicone brake fluids contain inhibitors which help prevent the corrosion caused by the water, but there is more chance of it boiling under relatively light braking.  DOT 5 is more compressible and can make the brake pedal feel spongy. Very importantly DOT 5 aerates very easily and is not suitable for any car fitted with antilock brakes.

Silicone brake fluids are not all bad – they have a very long life and are ideal for military vehicles which may be stored for many years, but must be ready for action at a moment’s notice. They are also favoured by classic car owners, because they don’t damage paintwork and the cars are only brought out for special occasions.

You can’t mix silicone and glycol (sometimes referred to as synthetic) based brake fluids and if you want to change from one type of fluid to the other you will need to flush the system before changing.

© Vanessa Guyll, August 08