Bottom line being that a moisture test at the reservoir may not be representative of moisture content in the wheel cylinders for whatever reason even though it does disperse through the system to some extent.

I'm not sure its because water is heavier though - the stated specific gravities seem to show that brake fluid is slightly heavier - but those are temperature dependent - and do not appear to be measured at the same temperatures.

Even it if moisture content in the reservoir was representative of moisture content in the wheel cylnders though, there are other issues besides moisture content - direct contamination and fluid deterioration.

I did not find that the condition of the fluid in my reservoir was representative of what was going on in my wheel cylinder on by 37 Ford, or my 60 Vette. Both are open and can absorb moisture at the reservoir, and I suspect at the wheel cylinder seals too. But if not attended to (learn the hard way) the moisture will cause corrosion, and that will release physical particles. On my C5 the clutch reservoir fluid was black. Using the ranger method repeatedly I was able to get it clear even when cycled. But it was not very long before it turned black again - I had to replace multiple components because I failed to bleed/flush that system. In my 60 vette and 37 Ford, the fluid was rust colored.

The attached image from 2022 shows my 60 vette wheel cylinder. Those brakes were working. Yet when the boot was peeled back, this is what was behind the boot. There was no visible fluid or leaking outside of the boot, but clearly there was some seepage there. Meaning moisture could enter.

 

But if you have a diaphragm in the reservoir cap then only the small volume of air trapped in the reservoir can introduce moisture and once that small amount of moisture has been absorbed by the brake fluid there isn't fresh air to replenish it unless the reservoir cap is frequently removed. The rubber in the diaphragm (like other rubber in the brake system) is permeable to a small degree but it isn't like the reservoir vent is open to atmosphere on modern brake systems.

I briefly read through the links and they do support that moisture testing is useful and that brake fluid does migrate through the system, it isn't stagnant. Otherwise how would discolored fluid make it to the reservoir?

I'm sure it is possible to have more moisture in the rest of the system than in the reservoir but it is most likely that they aren't way off. I.e., you are unlikely to exceed 3% if your meter reads less than 1% or 2%. The next time I replace my fluid I'll check for moisture both in the reservoir and what is bled from the wheels but I suspect there won't be much of a difference. In my case I live in a dry climate and I've never even hit 3% but wind up changing the fluid because of discoloration.

 

This! ^^^^^^See what happens.

I've been running an experiment for day or so now - to see if moisture added in one end of a system will "disperse" in the fluid to the other end. My "system" is a bent tube as pictured. I filled it with fluid and tested it at both ends and in the bottle it came from - its measuring either 0% or <1% - it fluctuates. But then I added a "slug" of water - about an "inch" of length in the tube. I did this by adjusting the position of the tube to leave a gap on the right hand side of about an inch, then added the water slowly to the left side until it brought up the fluid level on the right right side to the edge of the tube.

The result initially was an area of cloudiness as shown. The idea was to see if the moisture would disperse resulting in an increase in moisture on the right hand side - so yeah it must go down hill and uphill. So far, between 10:14 AM on 3/29 and about 6 PM today, 3/30, the moisture content on the right hand side has not changed. Its 0% and sometimes <1%.

But there have been some changes. The cloudy area continued to hug the left top, it did not visibly sink, but........the length of the cloudy area shortened over a period of hours, and as of this morning, the cloudy area was gone. I think this because the it takes some time for the fluid to actually absorb that water - I did not do anything to facilitate mixing. Oh, yeah, I can measure moisture on the left side, and off the scale - so its concentrated there for sure. But has not reached the right hand side. Is it at the bottom now?? IDK.

One more thing I noticed - in the vicinity of the cloudy part, there are tiny bubbles forming on the inside walls of the tube that extend well below the cloudy area. The right hand side shows no such bubbles. I conclude that this is evidence that the presence of moisture (and the water also probably had some O2 content). at least as far down as the bubbles are appearing. That is, after all, the only difference between the left and right side. More evidence its "dispersing". And I also noted the fact that those bubbles are moving - as they form on the sides of the tube and release - they move upward. This creates some fluid motion which I think may facilitate dispersion. Vibration, temperature fluctuation, and brake applications will probably facilitate that. Similar stuff is going on in closed brake system.

I was hopin' to be able to say that the moisture fully dispersed to the other end of the "system" - but not yet. I'll leave it go for a couple of days.


10:16AM on 3/29 The cloudy area is where I slowly added a little water.



1:12 PM on 3/39 - Cloudy area is shrinking - I think as the fluid absorbs the water. You can see the bubbles forming on the left side but not on the right side. The attached video shows a close up the bubbles rising.


1:12 PM 3/29 close up of bubbles. None on the other side.




3:15 PM oon 3/29 - Cloudy Area is still shrinking. Moisture content on left side is off the scale, content on the right has not changed - 0% or <1%



9:03 AM today, 3/30. Now the cloudy area is completely gone. And, there are NO bubbles anymore. Moisture level on left side is off the scale. On the right side unchanged. You cant see the part in the vice but that fluid remain clear with no visible changes.

 

Nice! Thanks for the quick demo!!

 

Yea, it will take some time. The amount of water you used is definitely beyond a worst case scenario and should easily migrate with some time since it is much easier to disperse in your test than in a brake system with lines. It will also be interesting to see if water separates in the fluid. Brake fluid is hygroscopic on purpose (except DOT 5) so it doesn't settle out and corrode localized areas. 3% is likely conservative with respect to coming out of solution but enough to significantly lower the boiling point.

 

If you want to speed up the dispersion ... pressurize the tube, heat the opposite end to 300 - 400 *F, and then pulse the pressure on/off

 

Unfortunately not being a chemist, I can't necessarily explain everything I have observed, and I don't necessarily even have the correct nomenclature. But I think when I initially added the water, that cloudy area that first appeared, immediately, was a condition where the water was separated - partially mixed - but not absorbed. The only interface between the water and the fluid is the cross section of the tube - but even though I added the water slowly, the agitation caused some initial mixing which caused the cloudy appearance. But it wasn't really "absorbed" yet (I think). And I think it is significant that the cloudy area continued to remain at the top - it did not sink into the fluid. I think that's because, as noted I from the relative specific gravities, fluid is actually slightly heavier. But even though it did not sink, it did shrink - and that I think is means its being absorbed. Since the the cloudy area disappeared completely, I think all of the water was absorbed. No visible evidence of any difference in the fluid now, even though the concentration of moisture on the left is very high, and on the right has not changed.

I tried to look up the difference between mixing and absorbtion (that is how I spelled it). AI said oh, yeah - there a difference. But when I look at the references, they are explaining the difference between absorption and adsorption. Note the differences in spelling - details matter. And it appears that the term absorbtion, which I thought was the process of absorbing something, does not exist. I need a chemist to explain what I am observing. I only know as fact that there was cloudy area that seem to "float" on top - and over a period of hours it shrank, from the bottom up, and now there is no visible evidence of a fluid change. And, the concentration of moisture remains high where I added water, and unchanged on the other side.

By the way, the specific gravities are at specified temperatures and change with temperature. And, I don't think they use the same temps for those specs. The water was colder than the fluid when I added it, but the cloudy area still remained at the top left as it shrank.

 

Here is an update, 8:30 AM on 3/31. This morning I removed the tube from the vice so I could inspect the bottom part. When I do that, there is of course slight (very slight) agitation - I try to minimize that but I keep the tube positioned so the fluid does not spill out of either end. There is no visible separation of fluid and water at this time. But there was single, tiny air bubble there at the bottom. That was not there before. Is that evidence of dispersion? Something leaching from the plastic tube? Who knows? Concentration remains high on the left side where the water was originally added. Its unchanged at <1% on the left right side as it was before any water was added.

All Below are 8:30 on 3/31

 

I have had no issues with a vacuum pump flush on my C8. Just disconnect the battery, remove the screen from the fluid reservoir, install the gravity feed bottle in the filler neck and vacuum away. As a final step, before reconnecting the battery, pump the brake pedal until it's firm. There can't possibly be a secondary reservoir in the system that isn't fed by the main reservoir and has access to air. Using a vacuum pump while the brake system is connected to the battery is definitely no no, though.

 

Really appreciate the semi-scientific testing, @Andybump. Bottom line though is that if you replace the fluid every 5 years, as the manual calls for, you're unlikely to have issues with significant water content or wear particles to cause any issues. If you're heavily tracking your car and running racing fluid, you're probably changing more frequently anyway.

 

Yes all true. I do replace mine at the specified intervals including the flush.

I did the test because I wanted to confirm the the the absorbed moisture will disperse, and how fast. And also to see if it separates as some articles seemed to say. But not because its a basis for delaying the fluid flush on schedule. But moisture is not the only concern as pointed out in several posts.

 

3 days and some hours later, and it now appears that the moisture has dispersed in the tube. No sign of separation or settling out. It now reading 3%. In retrospect I should have capped off that end since it could, I suppose, absorb moisture directly from the air. And maybe I should have set up a control rig with no water added.

I'm just trying to investigate the dispersion of the moisture - I'm not saying anything about not following the recommended fluid flush schedule - I follow it.

This is at 5:30 on April 1.

 

While all this brake fluid stuff is cool, doesn't it bother anyone that two of the adjustable wrenches face one way and the other three the other way????

 

At least, they look mostly tightened all the way : P

 

What an embarrassment! I try to keep them aligned.

 

The wrenches have been aligned and adjusted. Also attached is the rest of the "collection" hanging on the wall. There are two types displayed. The bottom row are dropped forged steel. The top row are the new and improved Crestaloy - some strength but aren't as thick and heavy (they said). The jaws can't be closed while on the display - there is peg there to keep them nice on the display. The prices are still on the display - in pencil I think. Crestaloy costs more. Also you can see there is blue metal box that says Crescent Tools. That is an incredible parts kit I ran across that includes replacement jaws, spiral thumb screws, shafts, and screws for most every size made. Crestaloy was introduced in 1930 (wow) - but obviously Crescent made both for awhile - because their display has both.

To keep this roughly on topic, I'll mention that these wrench sets will fit most any hex head fastener found on the C8.

 

I purchased a Crescent tool kit. It had sockets, wrenches, screwdrivers, etc. in it but not one Crescent adjustable wrench...