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Brake fluid - flush and replace. Why?

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NutmegCT

NutmegCT

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I've read in many places that brake fluid absorbs water and needs replacing "regularly". Water in brake fluid can corrode lines and lower the boiling temp.

The brake system is a sealed system ... how does water get into the fluid? I can't imagine it's because of the tiny bit of air at the top of the brake fluid reservoir.

Thanks.
Tom M.
 
Just like a fuel tank, the metal lines sweat inside from whatever they are made as the molecules of brake fluid have space enough to retain moisture. Never figured out how, but I bleed brakes every 3 yrs here in the Midwest. Hydralics get this moisture, like the, jaws of life, can be run off water in emergencies, drain and refill with fluid. Problem with water it gets hot and expands too quick to be retained. What I understand.
 
The master cylinder reservoir is vented. Each time the brakes are applied, the level in the reservoir goes down as the fluid goes to the wheel cylinders. Air enters the reservoir. When the brakes are released, the fluid returns to the reservoir, and the air is expelled. That exchange of air brings moisture in. In areas where humidity is high, it's a bigger problem. In areas where it is dry (California, Arizona) it's less of a problem.
 
John Turney said:
The master cylinder reservoir is vented. Each time the brakes are applied, the level in the reservoir goes down as the fluid goes to the wheel cylinders. Air enters the reservoir. When the brakes are released, the fluid returns to the reservoir, and the air is expelled. That exchange of air brings moisture in. In areas where humidity is high, it's a bigger problem. In areas where it is dry (California, Arizona) it's less of a problem.
Click to expand...

+1

Thanks for reminding me. My Civic is overdue for its brake and clutch flush (3 year interval).
 
Thanks gents. I can see how the air in the reservoir can mix with the fluid in the reservoir. But it sure seems a stretch to imagine the reservoir air going into the fluid throughout the whole system.

Perplexing.
 
Thanks John BN4, forgot all about the vent. Won't work if not vented.
 
Many newer braking systems have a flexible diaphragm on top of the reservoir, to reduce water absorbtion.

But according to this paper presented to the SAE, conventional brake fluid can suck not only water, but road salt, right through flexible brake lines
https://drive.google.com/open?id=0B2H2NJt34OffNWE5ZDE3NmQtYTViYy00NzI2LTkxODYtZjdhNzQ5YjYxZGY2
S61U9Qs.jpg


So even modern cars usually have a fairly short schedule to replace the brake fluid. This from the wife's Honda Accord workshop manual
Hl3GLep.jpg
 
Randall - thanks for those references. contaminants going right through the flexible brake lines and into the fluid. Wow. And the distinction between the Honda maintenance display, and Honda's own recommendation - amazing.


Who ya gonna believe!
 
Brake Fluid is hygroscopic, by composition it absorbs water. I googled 'why' and it seems to have to do with water not pooling in the system and with not lowering the boiling point.

Fuller explanation here:

[FONT=&quot]The hygroscopic nature of DOT brake fluid is actually an advantage in a major way.[/FONT]
[FONT=&quot]So where does this water come from?[/FONT]
[FONT=&quot]Glycol based fluids are hygroscopic which means they absorb water/moisture from the environment at normal atmospheric pressures at a rate of 2-3% per year. This process is exasperated in more humid conditions and climates.[/FONT]
[FONT=&quot]This water content finds it's way into the brake fluid via microscopic pores in brake hoses, seals, joints and seams. As we've learnt, water mixed with DOT fluid has an adverse effect on the brake fluid by reducing it's boiling temperature and therefore reducing it's performance.[/FONT]
[FONT=&quot]Here is how.[/FONT]
[FONT=&quot]As water enters the system, instead of pooling in low spots (such as the calliper), due to it's weight in comparison with brake fluid, it is dispersed throughout the whole of the brake fluid. This helps to keep the boiling point of the entire brake fluid high rather than having pools of water in the system which will boil much sooner than the rest of the brake fluid.[/FONT]
[FONT=&quot]It also prevents localized corrosion of internal parts which can be caused by water pooling in the brake system.[/FONT]
[FONT=&quot]In some cases, Mineral oil is used in braking systems.[/FONT]
[FONT=&quot]Unlike DOT fluid, Mineral Oil is hydrophobic and does not absorb moisture from the environment. This means that there are no wet or dry boiling temperatures to worry about, the boiling point stays constant and never drops. That's the good news.[/FONT]
[FONT=&quot]The bad news is that any water that does enter the brake system, via seals or microscopic pores in the lines etc., will effectively reduce the boiling point of the whole brake system to that of water - just 100°C. This is because as the fluid repels any water ingress, it causes it to pool at low points within the brake system, usually the caliper, since water is heavier than brake fluid it will settle at the lowest point. This is worrying because the fluid in the caliper is more susceptible to high temperatures as it's at the business end of the brake, where the friction is created.[/FONT]
[FONT=&quot]So, picking a less hygroscopic fluid, does not necessarily mean less brake fluid swap and corrosion.[/FONT]
 
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