Automotive Fluids - Lubricating Oils & Greases, Fuels, Coolants & Brake Fluids

KEW Engineering




How does Oil Protect?

Base Oils Explained

Oil Aging & Degradation

Additive Formulations

Oil Viscosity Explained

Oil Quality Ratings

FAQs on Motor Oils


Brake & Clutch Fluids




Disposal of Used Oils




Copyright 2009

KEW Engineering Ltd


Brake Fluids

The average motorist has very little comprehension of the importance of Brake Fluid as a SAFETY CRITICAL PRODUCT, or of the need to change it on a regular basis.  This lack of understanding is very dangerous because, in contrast to many other automotive products, if Brake Fluid fails a fatal accident could occur.

Brake Fluid is the medium for transmitting pressure from the brake pedal to the calipers or wheel cylinders where it operates the brakes.  The main requirements for a quality Brake Fluid are: -

    • High Boiling Point:  Heat generated by wheel brakes is considerable, up to 700C can be reached, at which point the discs can be seen glowing.  Some of this heat will find its way into the Brake Fluid via the calipers.  Obviously, if the Brake Fluid boils (converts to vapour) it will become compressible, thus the travel of the brake pedal will be taken up in compressing the vapour rather than activating the brakes, and the braking system will fail.

    • Balanced Viscosity:  Brake Fluids should not thicken excessively at temperatures down to minus 40C nor become too thin at the high temperatures generated by heavy braking.

    • Good Lubricity:  The Brake Fluid must provide lubrication for the many moving parts in a typical braking system over a long period of time.

    • Non-Corrosive:  Brake Fluids must protect a wide variety of metals and alloys in a braking system from corrosion. 

    • Compatible with Rubber:  Brake Fluid must not cause excessive swelling or shrinkage of rubber components which would lead to increased wear and break up of the seals which would lead to Brake Fluid leakage and loss of braking effort.

    • Stability:  Brake Fluid must retain all of the above characteristics throughout its life in the braking system.

Todays Brake Fluids are highly advanced mixtures of glycol ethers, glycol ether/borate esters and polyglycols, together with a package of additives to improve those characteristics necessary for a quality Brake Fluid.  But the main point is - Brake Fluids are synthetic chemicals and are NOT mineral oil nor related in any way to mineral oil - a common misconception.  No petroleum or mineral-based product should ever be mixed with or substituted for Brake Fluid.

Over time, demands on braking systems, and therefore Brake Fluid, have increased dramatically.  Disc brakes, ABS anti-lock systems and aerodynamic design reducing brake-cooling air have made operating conditions far more severe.  This together with automatic transmissions, which reduce available engine braking, has increased total stress on the Brake Fluid.  For these reasons, specifications for automotive Brake Fluid have shown an increasing trend towards higher initial boiling points, and more particularly, to retain those boiling points in service.

The properties necessary for quality Brake Fluid are controlled by the major Brake Fluid specifications ie the US Federal Motor Vehicle Safety Standard (FMVSS) DOT 3, DOT 4, DOT 5/5.1,  SAE J1703, and ISO 4925.  Many OEMs have their own Brake Fluid specifications while many national standards authorities also have their own, but as a general rule all are based on the FMVSS, SAE or ISO standards.  The main differences between them are summarised as follows:-



Boiling Point

C, min

Wet Boiling Point*

C, min


@ -40C, max









DOT 5.1




SAE J1703




ISO 4925




    •  *Wet Boiling Point measures how well brake fluid retains its boiling point in service.

    •  Very roughly, it equates to the expected boiling point after two years in use.

It is evident from the table that the main advantage of DOT 5.1 and DOT 4 is their higher boiling points as compared with DOT 3, SAE J1703 and ISO 4925.

Brake Fluid in Service

While minimum Brake Fluid boiling points are controlled by the specifications above they typically start out higher.  However, Brake Fluids are hygroscopic that is they readily absorb moisture from the atmosphere - a property which over time will reduce the boiling point and hence the vapour locking temperature.  The diagram below shows the effect of water on Brake Fluid boiling point.

Any brake system requires a relatively incompressible fluid to transmit pressure from the brake pedal to the callipers and wheel cylinders.  If vapours are present in the Brake Fluid, pressure transmission will be reduced or will fail completely.  The condition known as vapour lock occurs when Brake Fluid boils and forms vapour in the brake system, thus the brake pedal travel compresses the vapour and fails to operate the brakes.   Boiling can occur quite easily when the brakes are being worked hard on steep descents or when towing a load such as a caravan.

The presence of even a small quantity of water can result in corrosion of the different metals used in braking systems eventually leading to brake failure through leakage.  The surfaces of the cylinder become pitted from rust and so leakage results.  Moisture ingress is possible through the the flexible rubber hoses, the seals and the breather of the reservoir cap.  

Brake Fluid contamination by mineral oils and petroleum solvents, which are incompatible with Brake Fluid, will cause excessive swelling and softening of rubber seals and hoses, and may well lead to complete system failure.

Changing Brake Fluid.

Brake Fluids have a limited life, not only because of water absorption but because corrosion inhibitors and stabilisers are depleted over time.  Wear particles and rubber fragments will also slowly build up.  Unless the vehicle manufacturer recommends otherwise, Noria would recommend that Brake Fluid be changed at intervals of 12 to 24 months, depending on local climate.  In locations with cold and wet climates Brake Fluid should be changed at the shorter interval.

Storage and Handling.

Brake Fluid should be stored in its original container in a clean, dry location at or below room temperature preferably separated from similar storage of petroleum products or fluid materials used for maintenance purposes.   

Always use the original containers, which should be tightly sealed to avoid water absorption and be clearly marked. 

Under good conditions of storage, unopened containers of Brake Fluid will have a shelf life of two years.

Brake fluid is not intended as a top-up fluid - if the level in the reservoir is dropping it needs immediate investigation. 

Other Types of Brake Fluid.

The remarks above relate to conventional Brake Fluids that make up about 98% of the market.  There are two other types of Brake Fluid that should be mentioned:

Silicone Brake Fluids.

Silicone, or DOT 5.0 Brake Fluids are based on polydimethyl siloxane and are specialist fluids intended for racing applications and are dyed purple for easy identification.  Silicone Brake Fluids may be used in conventional classic car braking systems, as they are compatible with the standard brake components.  However, they do not mix with conventional Brake Fluids and should only be considered for use after a complete brake system drain and overhaul.  Mixtures of Silicone and conventional Brake Fluid may result in additives transferring between the phases thus resulting in reduced performance.

The advantages include:

    • A higher Boiling Point than DOT 3 and Dot 4, approx 260C

    • Not aggressive to paintwork

    • Not hygroscopic

    • Significantly longer service life (up to 3 times that of conventional fluids)

    • A very high VI, >300, giving a consistent pedal feel irrespective of the ambient temperatures

The disadvantages include:

    • A very high bulk modulus with a tendency to aerate when pumped

    • Unsuitable for ABS systems

    • Does not have any brake manufacturer's OEM approval

    • Water that enters the system will settle in the wheel cylinders

    • Breakdown of the fluid can cause abrasive silica material is often quoted as a negative but for this to happen the fluid needs to burn in the presence of oxygen and unless your brake discs are glowing red hot for any length of time this disadvantage is not a concern

    • A tendency to leak more easily than standard brake fluids

As a result of the slightly compressible nature of of Silicone Brake Fluid, ideally the master cylinder should be replaced with one that is the next size up in diametre to prevent a longer pedal travel.


Mineral Based Brake Fluids (LHM Fluids)

 A few vehicles in the past, including Citroen (hence the LHM title, as in Liquide Hydraulique Mineral) and Rolls Royce, used a mineral or petroleum based central hydraulic system for the suspension, which also powers the brakes.  To do this the brake system is fitted with special rubber components that are compatible with petroleum products.

LHM is NOT compatible with conventional brake systems, nor are conventional Brake Fluids compatible with systems requiring mineral oil.  Failure to use the correct fluid may result in total brake failure.