Brake and Clutch 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: -
Heat generated by wheel brakes is considerable,
up to 700ºC 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.
Brake Fluids should not thicken excessively at
temperatures down to minus 40ºC nor become too thin at the high temperatures
generated by heavy braking.
The Brake Fluid must provide lubrication for
the many moving parts in a typical braking system over a long period of
Brake Fluids must protect a wide variety of
metals and alloys in a braking system from corrosion.
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.
Brake Fluid must retain all of the above
characteristics throughout its life in the braking system.
Today’s 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. However, some older brake systems were specified for mineral oil use
by the manufacturer. Check your handbook. Citroen was one example that
used to specify a mineral oil braking system.
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 be able 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 OEM’s 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:-
@ -40ºC, max
*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
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 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
calipers 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.
The presence of even a small
quantity of water can result in corrosion of the different metals used in
modern braking systems eventually leading to brake failure through leakage.
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, new, unopened containers
of Brake Fluid, with their foil seal intact, will have a shelf life of two
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 Brake Fluids
are based on polydimethyl siloxane and are specialist fluids intended for
racing applications. Silicone Brake Fluids may be used in conventional
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.
However, because they do not absorb moisture as readily, they tend to last
much longer in service. In addition, to the enthusiast that values his/her
paintwork, they are much kinder than the normal brake fluid formulations.
To this end, classic car enthusiasts often use this. However, be aware that
the Bulk Modulus of silicone fluids is lees than that of normal brake
fluids, and because of this slight compressibility, it can lead to a
slightly spongy pedal feel. This is because ideally, one would work with
differing master and slave cylinder dimensions to overcome this property,
but since so few OEMs are willing to invest in this, silicon has failed to
achieve any OEM or motor manufacturer approval.
Mineral Based Brake Fluids.(LHM
A few vehicles, including
Citroen and Rolls Royce, use a mineral or petroleum based central hydraulic
system, 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.
For more information please
contact your supplier or Noria UK Ltd. 01244 659381.
By Hugh Lundin of
Noria UK Ltd