Brake fluid explained

[oilman]

Brake fluid... Bit of a mystery topic!

To help dispel some myths and for some good solid general info on the mysterious world of brake fluids I decided to contact Millers Oils up in West Yorkshire.

Their Technical Director, Martyn Mann was on hand to give us some useful info… below is Martyn's article on brake fluids.

There is a degree of confusion regarding the specification of brake fluid and this article sets out to clarify the situation.

The Department of Transportation (DOT) classifies brake fluids to defined specifications. These specifications relate to their boiling points and chemical composition, both of which are important. All currently available brake fluids are covered by one of the following specifications; DOT3, DOT4, DOT5 and DOT5.1.

The laws of thermo-dynamics dictate that the energy from motion is turned into heat through friction. A braking system only works efficiently if the fluid remains incompressible. If the brake fluid boils, it turns to gas, which is compressible and the braking system becomes “spongy” or in extreme cases fails completely.

A brake system is not perfectly sealed and moisture can get into the system and be absorbed by the fluid. The effect is to reduce the boiling point of the fluid, which reduces the efficiency of the braking system, as described above.

The DOT specifies two reference tests for brake fluids.

* Dry boiling point - the boiling point of fresh fluid

* Wet boiling point –the boiling point once the fluid has absorbed moisture (representing brake fluid after time spent in a real situation).

There are two main types of brake fluids:

* DOT 3, DOT 4, Super DOT4* and DOT 5.1 which are based on poly glycol compounds.

* DOT 5, which are based on Silicone.

Note the two types of fluid are not compatible and must not be mixed in a braking system.

SILICONE BRAKE FLUID (DOT 5)

Silicone based DOT 5 was originally introduced to give higher temperature performance over glycol DOT 4. Silicone fluid also has other advantages, it does not damage paintwork and it does not absorb water.

However, silicone fluid is a poor lubricant and does not lubricate ABS pumps as well as PAG fluids. It is also more compressible than PAG fluids, which can result in a sluggish or spongy pedal/lever. It therefore requires special design considerations in braking systems. Further, because it does not absorb water, any water remains as globules, which can pool in low spots in the system and cause corrosion. This water can vaporise when heated under heavy braking giving a disastrous effect on braking efficiency.

DOT5 fluids are not recommended for motor sport applications.

POLY GLYCOL BRAKE FLUIDS (DOT 3, 4 AND 5.1)

Glycol based DOT 4 fluid is the current mainstream brake fluid, and you will see that the specification is considerably better than DOT 3 which it replaces.

DOT 5.1 has higher specification still and is for fast road and occasional track day use. It has a similar spec to DOT4 for the boiling point (>260) but is a lot lower viscosity @-40C typically 900 centistokes (compared to 1500 - 1800 centistokes for DOT 4 and super DOT 4).

Listed in the table below, are the minimum dry/wet boiling point specifications for each DOT level.

BOILING POINT:
DOT 3 - 205°C (dry) / 140°C (wet)
DOT 4 - 230°C (dry) / 155°C (wet)
DOT 5 (silicone) - 260°C (dry) / 185°C (wet)
DOT 5.1 (PAG) - 260°C (dry) / 185°C (wet)
Super Dot4 * - 300°C (dry) / 195°C (wet)
(racing brake fluid)

* Super DOT4: The main difference between DOT 4 and Super DOT 4 is the dry boiling point. Normal Dot4 is >260C whilst Super DOT 4 is more like >310C

With thanks to Martyn Mann - Technical Director Millers Oils.

Cheers

Guy.

Opie Oils

[benny hedges]

cheers for that.

a question though.... as super dot 4 is obviously superior to normal dot 4, why is it commonly not regarded as suitable for road use?
take for example a road going bike (or car) that is used for occasional track days, where the brakes are given a proper work out and often suffer from fade 2/3 through a session when using ordinary dot 4 fluid - with super dot 4, braided hoses etc this doesn't happen - so why don't all vehicles use the higher spec fluid as standard?

is it just a cost thing?
can't see why tbh as it's not like you change brake fluid every week, so if the higher spec fluid is ok for road use, guess what will be going in my bikes!

you got any offers on super dot 4 fluid then?

[Jaglifter]

Benny

From memory, its cos Dot4 absorbs moisture more readilly. So you have to change it more often.

[oilman]

cheers for that.

a question though.... as super dot 4 is obviously superior to normal dot 4, why is it commonly not regarded as suitable for road use?
take for example a road going bike (or car) that is used for occasional track days, where the brakes are given a proper work out and often suffer from fade 2/3 through a session when using ordinary dot 4 fluid - with super dot 4, braided hoses etc this doesn't happen - so why don't all vehicles use the higher spec fluid as standard?

is it just a cost thing?
can't see why tbh as it's not like you change brake fluid every week, so if the higher spec fluid is ok for road use, guess what will be going in my bikes!

you got any offers on super dot 4 fluid then?

Super DOT4 is fine for road use, we deal with a lot of road cars and bikes that use the Motul RBF600 daily, then use the same on track at the weekend. Okay, it absorbs water slightly quicker, but not enough to make it a problem (it's not like people change it after just one track day or anything like that). I'd suggest using something like the RBF 600, Millers 300 Plus or Fuchs Pro Race and then change it probably annually. Bikes don't need too much fluid, so it's not expensive to change annually.

It's basically a cost reason why manufacturers don't use a super DOT4 in all vehicles, most are a lot more expensive than normal DOT4s and aren't really necessary on a road only standard car.

It would have been sensible to have an offer on brake fluids as we're posting this offer, but I'm not involved in marketing. There's a free shipping offer coming up in the next few days, so that should help. Get registered on our site and we'll email you the details when it starts.

Cheers

Tim

[benny hedges]

nice one - sort of confirms what i expected - just heard a lot of (sunday) riders say you cant use racing fluid on the road!
thanks - i'll invest.

[oilman]

Probably the same people that say bikes must only use mineral oil.

[tony.mon]

nice one - sort of confirms what i expected - just heard a lot of (sunday) riders say you cant use racing fluid on the road!
thanks - i'll invest.

But does it make you quicker?
I invested in a racing keyring a while back- haven't noticed that advantages yet, but then it's been a bit snowy.

Roll on spring!

[benny hedges]

Roll on spring!

sounds uncomfortable, but dont spose it mattress

[oilman]

But does it make you quicker?

The better you can brake, the faster you can go