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  #1  
Old 15 Dec 2010
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Brake fluid explained

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
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Old 16 Dec 2010
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Nice one

The main point for me is that the non-silicone brake fluid is hydroscopic. This is useful to know because its worth about 200 points at Scrabble, but also because it is a real reason to change the fluid after 2-3 years. The fluid absorbs water. The water boils at 100 C and the steam is compressible. The brakes go spongy quicker with old fluid, but at 3 years plus it will start to become critical.

The other good way to wreck a brake system is to use hydraulic oil instead of brake fluid. This stuff swells seals makes a real mess.

In my past job I dealt with hundreds of claims. The top four reasons to reject a claim were old fluid, wrong fluid, badly bled systems (usually dealers using vacuum fill) and welding damage to ABS electronics. Don't try these at home!

Andy
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Old 16 Dec 2010
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Hydrocracked is another good one for scrabble.
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Old 16 Dec 2010
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Thanks

Thanks,
I always wondered the whys and wherefors (spelling?- Scrabble legal?) about brake fluid as I struggled to get DOT4 in Africa. The bike stated DOT4 but only DOT3 was available. It worked until I could replace it a while after.

The info will be kept in the 'back of the head' filing cabinet for future use, Cheers
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Old 16 Dec 2010
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Quote:
Originally Posted by geoffshing View Post
Thanks,
I always wondered the whys and wherefors (spelling?- Scrabble legal?) about brake fluid as I struggled to get DOT4 in Africa. The bike stated DOT4 but only DOT3 was available. It worked until I could replace it a while after.

The info will be kept in the 'back of the head' filing cabinet for future use, Cheers
Glad the info is of use.
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