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Brakes and fluid


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well going off isn't quite the right word.

As with most miscible liquids it does degrade over time to some extent, however, more importantly, brake fluid is hydrophilic which means it attracts water.

for a fluid in an enclosed system it seems rather good at this for some reason- if you don't change your brake fluid for a couple of years you will find that a significant % of the fluid is water (1% of soemthing silly like that). The dynamics of water make it totally unsuitable for brake fluid. Water increases fade susbtantially, reduces efective line pressure on the calipers and decreases pedal feel substantially.

if you are completely clearing the system of DOT 3 or DOT 4 fluid and are prepared to replace brake lines then swapping to silicon based fluid may be a good idea.

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Sorry to contradict a little but important with brakes.

The word you are looking for Pug is "Hygroscopic"

Silicone fluid is now banned in international events due to the following reasons.

1. Normal brake fluid is hygroscopic and as such as Pug correctly says, attracts and absorbs water, it does however also mean that the water absorbed is evenly distributed throughout the entirety of the fluid rendering small amounts harmless.

Silicone fluid does not absorb water and any water / moisture in the system (and there is plenty in any system not constructed in a vacuum) then congregates together and forms water pockets, now this is fine as long as it stays cold. I.e. you don't use your brakes, but as soon as you start using them and adding heat to the equasion the water pocket can turn to steam at some stage and you then instantly get air in the system and an instantly soggy or non existent pedal. Not good :o

This also happens with normal fluid but at a much slower rate as the water is evenly distributed in the fluid and turns to steam at a much slower rate so your brakes fade slowly and warn you as you loose feeling.

2. Dot 3 - 4 - 5 etc are different specs and sometimes a DOT 5 will kill seals in a system designed for DOT 3 etc, so not always good to "upgrade".

The best way I have found to drain / change fluid is to open all bleed nipples, fit a length of pipe to the ends with the other end in a jam jar etc and lightly pressurise the fluid reservoir with a couple of psi, do this until the system is empty and then purge it with new fluid in the same way.

You may have to lock off individual nipples as you go to make sure all fluid gets out.

Good luck,


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