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User avatar
By oilman
#2790969
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. 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
User avatar
By oilman
#2794125
I don't think it actually says it's a super DOT4 as technically it's just a very good DOT4. DOT3/4/5/5.1 are proper Department of Transport (US) terms, but Super DOT4 is more of a marketing one.

Cheers

Tim
User avatar
By Settez
#3277323
Spiral45 wrote:http://www.tegiwaimports.com/1l-motul-r ... fluid-260/

So as someone just mentioned, this ^^ rbf 600 is a good dot 4; ok.

For road use and seldom track use this should be bang on then? Or should I get a 5.1? Not sure on cost difference.

Cheap also. Will two 500ml bottles be enough?

Going to combine with my HEL braided lines.

Thanks
This is a god question, old but still good :lol: Anyone care to answer?
User avatar
By oilman
#3277342
A normal DOT5.1 costs a little bit more than a DOT4, perhaps a couple of quid a litre, but there is a big difference in the price of a standard DOT4 and a performance DOT4. A normal one is roughly £5-7 per 500ml, performance ones are usually at least £12 per 500ml, some like the Castrol SRF are over £20 per 500ml. When I say usually over £12 per 500ml, that is a loose rule of thumb, we've got a few like the Gulf RF800 and RF1000 and the Millers 300 Plus that are on offer and make them sensibly priced performance options.

In a lot of cars, a 5.1 is okay for some track use, but it seems that over the last couple of years that people are saying more often that their brakes are boiling the fluid, I guess brake material has changed a little. I'd look at the Gulf RF800 or 1000 if you want really good, sensibly priced brake fluid.

http://www.opieoils.co.uk/c-450-brake-f ... fluid.aspx

Cheers

Tim
User avatar
By billster
#3284049
Is there any difference between the DOT 3,4,5,5.1 etc in terms of how often they have to be changed?

I'm looking at getting a DOT 4 racing fluid as I might well be doing a couple of track days.

What I don't want to have to do is have to change it in 6 months though.

Should all modern fluids last a couple of years?
User avatar
By oilman
#3284125
A DOT4 racing type fluid is good for about 2 years according to the manufacturers, a DOT5.1 is usually about 3 years. Some people are paranoid about brake fluid on track and change the fluid every couple of races, but it's not necessary, in most cars running the super DOT4 fluids, 2 years is fine, perhaps longer as well.

Cheers

Tim
User avatar
By SeanEP3
#3435345
which of your fluids would you recommend for me?

300bhp civic. mainly road use, odd track day, and off to the ring in october.

4 or 5.1?

also, any difference in the brands? as a fair bit of price difference

thanks
User avatar
By oilman
#3435644
Hi

The best fluids for the brakes and clutch are the Castrol SRF, Motul RBF660 and Gulf RF1000. The Motul RBF600, Gulf RF1000, Millers 300 Plus and Fuchs Pro Race are close to those, but don't have quite as high boiling points. The next step down (in performance terms), but still an upgrade over standard are the Gulf Racing 5.1, Motul DOT 5.1 and Castrol React Performance. Any of the other DOT4 fluids we have are fine to use as a standard choice.

http://www.opieoils.co.uk/c-450-brake-f ... fluid.aspx

Cheers

Tim

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