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Less than 1 liter of brake fluid for change?


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I changed the brake fluid using a motive bleeder and super blue fluid. I did NOT flush the clutch. All went very well and very easy but I only used maybe 85% of 1 liter of fluid. Is this OK or did I not flush enough fluid from the lines...the fluid did run bubble free and blue BTW. I have a 2000S Manual tranny.

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Even when

...

I did a full flush at all 4 brakes and

...

I bled the clutch multiple times (because I'm an idiot and left the clutch pedal DOWN) and

...

I spilled several ounces on the ground(because it is really hard to reinstall the clutch slave cylinder - during a clutch change - without bleeding all over the place and compressing multiple times)

...

even then I used less than 1 liter.

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  • 2 weeks later...
"...  Now i'll have an extra liter for the future since I only used the one! :cheers:

Hi,

I'm not sure you have opened the 2nd Litre of fluid, but your post reads as if you may have. FYI, Brake Fluid is Hygroscopic, meaning that it pulls moisture from the air. As such, once it is opened and exposed to the air, it's moisture content can raise significantly in literally a matter of minutes.

Therefore, any container of Brake Fluid which has been opened, and resealed, has a shelf life of less than one month. After that, it's moisture content is so questionable that you should not use it. Topping up from such a container will only contaminate the fluid in your Brake System. Even an unopened container should be kept no longer than 6 mos. as their caps aren't the best seal and the presence of air in the container (except Motul which top-fills their containers with Nitrogen).

This is why you need to change your Brake Fluid every two years. A recent study by the SAE, testing 1500 cars bought new 18 mos. ago found an average of 4% moisture content in their Brake Fluid (and a high of 7%!). To put this into perspective, Castrol LMA Brake Fluid has a DRY (<3% moisture content) Boiling temp of 550°, but a WET (>3% moisture content) Boiling Temp of 218°. That means if you are using Castrol LMA w/ a 4% or greater moisture content, you may as well be running pure H²O through your system so far as Braking efficiency is concerned. In addition, flushing the system every 2 years removes any accumulated corrosion and will make your seals, lines and calipers last much longer.

Happy Motoring!... Jim'99

Edited by MNBoxster
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Hi,

I would never trust it to last 2 years even unopened. We're not talking about hermetically sealed Nuclear Waste Containers here.

The Plastic Bottle and cap just aren't that good, even with the foil seal. Also, while the bottle itself won't allow the fluid to permeate, it isn't that good when it comes to air/moisture.

Granted, if you buy from a shop, you don't know how long or under what conditions it was stored before you bought it either, but it's a better bet that it'll be uncontaminated.

Even the air pocket in the bottle can contribute to the contamination, not only from moisture, but from oxidation (Glycol based fluids do react with Oxygen). This is why Motul fills the 'air' cavity with Nitrogen and why they use a metal container. Is it worth the extra $$ ???

Happy Motoring!...Jim'99

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Jim, Are you simply concerned that a sealed metal container *might* allow some moisture contamination, or do you have actual knowledge that this is the case?

Note that Porsche thinks the brake fluid must only be changed every two years, even though the brake fluid reservoir is not very well sealed to the outside atmosphere. Given that, I am very comfortable using brake fluid from a re-sealed container after a few months, and storing unopened brake fluid almost indefinitely.

Also note that Ate Super Blue Blue has a wet boiling point of 420 degrees F which is probably adequate for a street driven car.

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HI,

I have actual knowledge of open brake fluid containers becoming moisture laden after only a few weeks. Of course, ambient humidity has a lot to do with it. In AZ I wouldn't be so concerned.

I have no disagreement with Porsche's Service Interval of 2 years (for regular Street Use), but this system has a vent and the moisture can come and go from the system. No system can be sealed, so there will be some contamination. My concern is not giving the process a helping hand by starting with bad fluid. But, the Interval is inadequate if you are tracking the car at all. I flush and Bleed the brakes on my Formula Vee before every race/track session (reverse power bleeding).

While the fluid you mention has a high WET boiling point, there's still a couple issues if the moisture content exceeds 4%.

First, even though the fluid won't boil, the moisture in it will. This moisture effectively turns to steam, a gas. And we all know that a gas is much more compressible than a fluid. So, best case, less of the force you impart to the pedal actually acts on the caliper, while much of it is used to compress the Steam Bubble in the system. Also, don't forget, water is heavier than glycol based fluids. It will migrate to the low spot in the system which is usually right at the caliper itself. This has two problems, first it is very close to the hottest part of the system - the caliper (which can reach temps of 1,000°+) and so will boil very quickly and second, the corrosion in the steel pistons and bores is much faster than the corrosion to the nickel/zinc coated lines.

Finally, at 4%+ moisture content, you are accelerating the corrosion to the caliper bore and pistons considerably. Hope this helps.

Happy Motoring!...Jim'99

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After doing some research, I am convinced that there is much too much paranoia floating about regarding moisture contamination of brake fluid. Yes, it does absorb some water, and yes that does lower the boiling point. And yes, brake fluid should be changed every year or two to prevent corrosion (because the corrosion inhibitors in the fluid break down over time). [reference]

But consider this - say you used half a can of brake fluid and resealed it when the relative humidity was 100%. Assuming the fluid absorbs all the moisture in the air, what would the water content be? The math goes like this: At standard temperature and pressure (25 degrees C, 1 atm) a half a liter of air contains about .02 Moles of gas molecules. Under these conditions, air saturated with water contains about 3% water molecules. That gives us .006 Moles of water molecules. Given a molecular weight of 10 for H2O, that equates to .06 grams of water in the air in the can. This represents about 0.012% water (by volume) in the half liter of brake fluid.

This probably explains why the side of a can of Porsche brake fluid simply tells you to reseal it well, rather than telling you to dispose of it. See Toolpants' Porsche brake fluid pic

Oh, and by the way, the boiling point of water *changes* when it is in solution with other compounds. For example, the Ethylene Glycol in anti-freeze both lowers the freezing point and raises the boiling point of the coolant. [reference] The definition of the boiling point of a solution is the point at which gas bubbles begin to appear. Gas bubbles in "wet" brake fluid don't begin to form at 212 degrees F, they begin to form at the "wet boiling point".

Also note that water absorbed by DOT4 brake fluid is in solution, and will not "migrate to the low spot in the system" as it would in DOT5 silicone brake fluid which is not water soluble.

At $10-$15 per can for a good brake fluid, it makes sense to use a new can whenever flushing the system and to flush frequently if the car sees the track. I just wanted to point out some of the misconceptions which seem to be pretty common.

 

HI, 

 

      I have actual knowledge of open brake fluid containers becoming moisture laden after only a few weeks. Of course, ambient humidity has a lot to do with it. In AZ I wouldn't be so concerned. 

 

      I have no disagreement with Porsche's Service Interval of 2 years (for regular Street Use), but this system has a vent and the moisture can come and go from the system.  No system can be sealed, so there will be some contamination. My concern is not giving the process a helping hand by starting with bad fluid.  But, the Interval is inadequate if you are tracking the car at all.  I flush and Bleed the brakes on my Formula Vee before every race/track session (reverse power bleeding). 

 

      While the fluid you mention has a high WET boiling point, there's still a couple issues if the moisture content exceeds 4%. 

 

      First, even though the fluid won't boil, the moisture in it will. This moisture effectively turns to steam, a gas. And we all know that a gas is much more compressible than a fluid. So, best case, less of the force you impart to the pedal actually acts on the caliper, while much of it is used to compress the Steam Bubble in the system.  Also, don't forget, water is heavier than glycol based fluids.  It will migrate to the low spot in the system which is usually right at the caliper itself. This has two problems, first it is very close to the hottest part of the system - the caliper (which can reach temps of 1,000°+) and so will boil very quickly and second, the corrosion in the steel pistons and bores is much faster than the corrosion to the nickel/zinc coated lines. 

 

      Finally, at 4%+ moisture content, you are accelerating the corrosion to the caliper bore and pistons considerably.  Hope this helps. 

 

Happy Motoring!...Jim'99 

 

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@ kevinC,

We have a couple disagreements.

But, to start, I concede that you are correct about water being in solution for DOT3, DOT4 and DOT5.1 Fluids and that only using DOT5 (which you should never do) will the water remain separate. I further concede that the Service Interval is due mainly to the breakdown of the corrosion inhibitors which are added to the fluid during manufacture, and not moisture related at all. The amount of moisture entering the system is determined almost totally by environmental conditions.

However, to begin, you assume that the fluid in the container is dry to begin with. To be sure, a certain amount of moisture does enter any non-metal container simply through diffusion. What amount would depend upon how long the container had been filled/stored, under what conditions, and of course the composition of the material used in the container.

But, while as you state, the boiling point of water changes in solution, it's effect on the boiling point of that solution (in this case Polyalkylene Glycol Ether based Brake Fluid) is to decrease it. And this decrease can be DRAMATIC - as much as 30% for some Brands of DOT3 and DOT4 for systems containing in excess of 3% water content, bringing the Wet Boiling Point to the 280°-350° range. A Street Car can easily experience Fluid temps at the caliper of 450°-500° for brief periods resulting in some fluid boiling. The relevant relationship is not the WET Boiling Temp/H²O Boiling Temp, it is WET Boiling Temp/Caliper Temp or more precisely Caliper Piston Temp which is almost the same (I have measured this with Temp Strips on the Pad Backing Plate and Caliper Body). A funny thing about Fluid Boils is that they change the chemical composition of the fluid, and though once cool will return most of their properties. But, each time the fluid boils, it's boiling point lowers a little further making it easier to boil the next time. The published WET boiling point is determined under Lab conditions at 3.7% Moisture Content and no consideration for the effects of Service Life are made (and Polyalkylene Glycol Ethers do degrade with time). BTW, your analogy of Ethylene-Glycol in a cooling system is not relevant as this is a Closed, Pressurized System and it is the increase in the pressure (often 1 additional ATM) which increases the vapor pressure and prevents boilover much more so than the chemical composition.

Fluid being Hygroscopic is, believe it or not, a design function of the fluid and a good thing. Again, moisture enters the system through diffusion (through the rubber hoses, seals and reservoir) as well as air which leaks past seals, all containing moisture. Another source is through condensation as the brakes are continually exposed to cyclic Hot/Cold. The fluid is designed to absorb this moisture and so to lower the local concentration which lessens Oxidation (corrosion).

It is totally subjective whether stressing the importance of insuring the best possible fluid for a change is paranoia or not. But, the SAE estimates that 50% of all cars 10 years old or older have Never had their fluid changed (can you imagine the numbers relevant to these cars). And, according to DOT figures, the average age of cars on the road today is 10 years. Still think it not important to stress proper, regular Maintenance? PEACE!

Happy Motoring!...Jim'99

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  • 11 months later...
Even when

...

I did a full flush at all 4 brakes and

...

I bled the clutch multiple times (because I'm an idiot and left the clutch pedal DOWN) and

...

I spilled several ounces on the ground(because it is really hard to reinstall the clutch slave cylinder - during a clutch change - without bleeding all over the place and compressing multiple times)

...

even then I used less than 1 liter.

Just curious, when do you release the clutch during the bleeding process? I closed the bleeder valve with the clutch pushed in and the clutch never released. The book is not clear on this procedure. I reopend the bleeder valve using the pressure bleeder and the clutch pedel was restored to its original position. I closed the bleeder valve and tested the clutch - seems to work fine but wanted to double check the clutch bleeding process. Did I bleed the system correctly by allowing the clutch pedel to be released and then close the bleeder valve?

Thanks - Allan

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When using the Motive I do not hold the clutch pedal down. But if you do, I think you need to pull the pedal up by hand.

The shop manual says to hold the pedal down, but we have never done it that way. I think you would need to do this only if you were replacing the master or slave cylinders. This is what Peter told me a few years ago.

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