What is the difference in compressibility between liquid water and the glycol based or ester based fluids? Is it even perceptible by foot compared to the difference in compressibility between those liquids and a gas?

 

It proportional with temperature and pressure which is different that of the brake fluid so I can't give you an exact set number. You could take one instance in time and measure the differences and then extrapolate values based on estimated conditions but for all intensive purposes on this forum, just take it for what it is:-). This is one reason why brakes can feel "mushy" when the brake fluid is highly diluted, it will be worse at some times based on said conditions. You'll have to look at a vapor dome and other data for comparison if you want to get more technical about it.

 

SJM- You got it right on. As I was reading through, I was wondering if anyone was going to mention that the water absorbtion rusts out the brakes lines internally.
I work for a dealer that sells BMW, Mercedes-Benz, Porsche, Saab, Volkswagen and Mazda. All the German brands have a set replacement time for brake flushes, usually during the maintenances. (20k, 30k...) I work the Mazda line and they do not have a set replacement time. Only to check it every 5K and replace as needed. We used to combine it with our maintenances as well, every 30K. My manager recently removed them from the maintenances saying that "Mazda doesnt recommend it at certain intervals, so we're not doing it". So now its a lot harder to sell these flushes to customers at seperate times or now saying when they do come in for the maintenance that its also recommended to do the flush and its gonna cost this much more...... We charge $120 I believe, (2 pints of fluid+labor). Alot of people dont believe in doing this, mostly because they have never known it was done or just think "hey, my brakes work just fine when I step on the pedal, what are they trying to sell me now that i dont need" The whole "stealership" thing...... We use a vacuum based tool to pull the fluid through to each caliper. When I empty it, not only do you get this dark fluid from the water absorbtion, but there's usually a sort of sediment at the bottom of dirt/metal particles. Just think of what that stuff is doing to your piston seals as they work back and forth everytime! And how well does the water provide lubrication to seals and pistons versus fresh brake fluid? Plus think that your brake fluid goes through how many heat cycles, just like trans fluid and engine oil, causing a break down of its properties over time. Why would you not change your brake fluid just the same as you do your engine oil, trans fluid, differential oil.....etc. The only time I do not usually recommend this is when its a high mileage vehicle that has never had it done before. I know that the bleeders have never been opened before and I dont want to risk breaking one off.

 

Sure, I was just wondering about a ballpark since it doesn't seem like it would really matter much compared to compressibility of gas phase water after you get enough temperature to boil it out, or if you have air in the lines.

Like they say ester base (like Castrol SRF, which does also have glycol) is more compressible than the glycol but I doubt many notice it too much in pedal feel. They also say the sillicone DOT5 is something like 2-3 times more compressible and is noticeable, although I have never driven a car with silicone fluid.

 

I honestly doubt a regular Joe in a street car with OEm type barkes could feel the compressibility difference between water, glycol and ester based fluids.

But only one of those fluids transitions to a gas at 212* and I would hope that ANY Joe could feel the difference between water, glycol, ester and steam.

And at 80* ambient temps, it doesn't take very much braking to reach 212*.

The bad thing is that when fluid boiling occurs, its typically when you really need your brakes ... and you don't have them.

 

honestly you may need a shop to do it. with the abs modules, it's possible to get air trapped in there resulting in less than optimal performance. could you get a decent pedal doing it at home? eh probably. but letting a shop with the equipment to purge the abs system is the best route.

 

Those people are idiots, they're killing their bearings. Brake fluid sits still basically and moves back and forth..........LITTLE different then engine oil bud.....


What I was TRYING to say was that the mechanic is a cheeseball to say "dangerously" dirty. No such thing. Brakes don't have dirty, old fluid and then one day 100% stop working, making it dangerous.

My brake fluid, as of a month ago, was 13 1/2 years old. I never once even had to top off the fluid. It was clean and just fine. So if his was "dangerously" dirty....then I should have crashed like 8-9 years ago.

Typical mechanic scare tactics to get work.............

My brake system now has steel lines and high performance fluid.....I won't change the fluid for years, no way......

.

 

Is your brake fluid still crystal clear as it was 13.58 years ago? Completely devoid of color or haze?

 

The 13 1/2 year old fluid was not that discolored......definitely could see through it.

Now its crystal clear because its brand new.

What exactly are you asking......?

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By a Motive-brand pressure bleeder and air in the lines and the ABS module won't be a problem. AND it only takes 15 minutes or so to do the whole system.

But, then again, bleeding with a buddy using the 'top off...pump...pump...pump...HOLD DOWN...release' and repeat method won't introduce air into the system either. Unless, you let the master cylinder do dry.

 

LITTLE different than engine oil bud ... irony is a bitch, ain't it?

I was just using your logic, that's all. Faulty logic ... but logic, none the less.

Again, using your logic and thought process, a fluid that's hygroscopic and is 100% saturated with water (brake fluid is hygroscopic and begins absorbing humidity immediately) and then sits in metal lines for 13.5 years will cause no degredation to metal parts.

Is that really how you think?

Do you ever change your brake pads? Metal backing plates will actually stop the car, so what's the point, right?

Interesting ...

Oh and in a solid, presurized system like a braking system, the fluid doesn't "move back and forth". It simply compresses, creating hydraulilc pressure. There is movement ... miniscual movement ... more at the master cyclinder and almost nothing at the caliper. But it doesn't flow through the lines like in a power steering system.

 

Well I have been following this thread for a day or two now. I had been waiting for the weekend to come to change my brake fluid and holy $hit was it nasty. A buddy helped my change it out and it looks like soda. We used the motive pressure bleeder and flushed the whole system out and neither of us could believe how nasty the fluid was.

 

Funny.. You can correct LS6427's posts yet you fail to ensure that your own are correct lol.

 

Here is a pic of the nasty $hit that came out of my system... I dont think its ever been changed.

 

Looks yummy. Did you notice a sizable difference in brake performance after you replaced the fluid?

 

The pedal only moves an inch, where it used to move 4 or 5.

 

Actually you're wrong. Each fluid has additives in it that create an RA value. A value that determines the fluids reserve alkalinity. As heat and time break down the fluid they become more acidic. The additives buffer the fluid and prevent it from acidifying as fast. As you can imagine you don't want acidic fluid inside metal lines, cylinders and calipers.

 

buy some speed bleeders and you can do it yourself

 

how sure are you that they didnt use used 5w30 for your brake fluid? hahaha

 

I change my brake fluid when I open the cap to check it (every time I change my oil) and it has a hint of a brown tint to it. I usually don't bleed the fluid out though. I use a turkey baster and suck all the fluid out of the reservoir and then top it back off. Yeah i know its a half *** way to do it but it better than doing nothing at all. So far I've done this ~ 5 times since the car was new.