Brake fluid, why is it so special ??

[KCinSB]

  Willing to be educated: After 47 years as an aircraft mechanic and amature home wrench; Why is Brake Fluid corrosive?
   I mean, we can send a man to the moon! Why haven’t we found a hydraulic fluid to use in a brake systems that is friendly to paint, does not corrode Alloy and Steel, and can take the Heat?  It is a tall order I know, but come on! It’s been +/- 100 yrs since the Loughead (Lockhead) Bros invented hydraulic brakes in Santa Barbara. We’ve had a million ruined parts and paint jobs ever since!  After DOT 1/2/3/4…..  Any Chemistry Majors here?  My latest example of the situation…………   

[Laitch]

Brake fluid is corrosive because it attracts H₂0 and elements in water can be corrosive. That's one of the reasons to flush brake systems periodically even when in storage.

[KCinSB]

  “ Brake fluid is corrosive because it attracts H20 and elements in water can be corrosive.“
Agreed,….. so maybe it needs to attract water? I can’t figure that out, and it seems like Expelling water would be a better plan. The oldest fluids I am aware of were castor oil-based hydraulic fluids and alcohols.. Again, the alcohol would attract and hold water. It’s the reason modern gas sitting in metal tanks will corrode the tank. Back then, without all the high tech engineering of today, it was the only way to combat cold temperature problems. But now? It just seems like something the industry doesn’t want to bother changing. No profit in it, for the effort required!

 https://www.scribd.com/document/290494058/The-History-of-Brake-Fluid

[Laitch]

You asked a question for which you already knew the answer. Maybe you had to look it up first. Anyway, here's a couple of questions for you.
Why, after millennia, do humans continue to buy junk then complain that it's junk? Is hope behind this? B. Franklin opined that those who feed on hope are likely to die of starvation.
Why, after millenia do humans neglect maintenance then complain about the consequences and cost of neglecting maintenance?

I doubt that the current standard of brake fluid comes down to a corporate conspiracy. It could be that consumers wouldn't want to pay the resultant cost of developing an alternative. That could be why the pace of development is slower than wanted, but it will improve eventually, unless we just quit stopping altogether.

[frankenduck]

DOT 5 is silicone and won't eat paint but ...

[Chaos]

Shimano disc brakes on bicycles use mineral oil.  Seems to work pretty well and it doubles as a laxative.

[frankenduck]

The K1600 uses mineral-based fluid in the clutch hydraulics.  But the seals in each system (clutch and brake) are designed for each fluid.

And mineral oil does not absorb water so it "pools" in the system.  Water boils at 212 degrees, a much lower boiling point than brake fluid. Since water weighs more than oil the pooling would occur where it gets the hottest - in the calipers.

[KCinSB]

That Youtube was actually quite informative, Thanks!  Higher Dot number is not better for the specific use.

[Laitch]

. . .  and it doubles as a laxative.

So does brake failure. It's a win-win, for the biking constipated. 

[frankenduck]

So does brake failure.

Both before and after the impact.

[Gabriel70]

  “ Brake fluid is corrosive because it attracts H20 and elements in water can be corrosive.“
Agreed,….. so maybe it needs to attract water? I can’t figure that out, and it seems like Expelling water would be a better plan. The oldest fluids I am aware of were castor oil-based hydraulic fluids and alcohols.. Again, the alcohol would attract and hold water. It’s the reason modern gas sitting in metal tanks will corrode the tank. Back then, without all the high tech engineering of today, it was the only way to combat cold temperature problems. But now? It just seems like something the industry doesn’t want to bother changing. No profit in it, for the effort required!

 https://www.scribd.com/document/290494058/The-History-of-Brake-Fluid

Hi

It’s a matter of priorities.
Brake fluid is hydroscopic as was previously mentioned.
We can ask a million questions about the world around us, unfortunately we don’t always have an answer or a satisfactory explanation.
Perhaps you have answered your own question.
It might be possible to create a brake fluid which meets your requirements. Is it necessary and what will it cost the consumer.
Brake fluid isn’t the only corrosive substance.

[daveson]

Some take home thoughts from what I've read about brakes many moons ago, so not confident, but still logical, I guess.

It's the nature of the beast that brakes get hot with use.  When you park your brick in the garage, as it cools down, condensation forms in the brake system and turns into water drops, and accumulates over time (if not absorbed by brake fluid) Water, like brake fluid, is a liquid and not compressable, not a big problem so far.

 Spose you're riding down a steep hill, as the brakes get hotter, let's say at about 100 degrees Celsius, the water vaporises from a liquid to a gas, and that is compressable. So you might run out of brakes, not a good thing.

Yep we have been to the moon, but a bottle of dot 4 is cheaper. Dot 5.1 has a higher boiling point, but dot 4 is standard for bricks.

[stokester]

Yes, DOT 3 and 4 brake fluid is hygroscopic in that it attracts and holds water molecules which will promote corrosion in the braking system.

To my knowledge DOT 5, silicone-based fluid was developed to address this issue but because it required the whole system to be thoroughly flushed and will tend to suspend very tiny air bubbles that made it extremely difficult to fully bleed.  This makes it a labor-intensive change from the DOT 3 or 4 glycol-ether based fluid.

Don't know why there hasn't been a fluid developed that would not absorb moisture and have the properties of the DOT 3/4.  Possibly a cost/value decision.
When I worked as a mechanic at the local Chevy store more than a few times I discussed the value of flushing the brakes when a customer had fluid the color of weak coffee.  This was frequently met with a response that it was not a requirement in my 19xx and why it should be now.  Truthfully it should always have been because of the absorption of water but with today's ABS, traction and stability control a failure may costs a significant amount.

For my Beemers a half day with my vacuum bleeder and a quart of DOT 4 at least every two years keeps everything working.