For those who track and need to flush on a regular basis I'm thinking of trying something and looking for feedback before attempting.

We need to unhook the battery or (apparently) the low brake sensor will see the low line pressure when you crack the bleeder and run the pump shooting fluid out.
Could we not take advantage of that to force fluid out and top the reservoir as necessary? No need to use a Motive bleeder or similar. We all know that vacuum pumps are a definite no-no.
I could see getting a code which would be easily cleared once the job is done.

Thoughts?

 

Not sure if it would pump 'continuous', not letting any air back in. You can try and report in, of course. I use SRF and don't bleed very often to be honest; I usually bleed when I do a brake job.

 

I just used a pressure bleeder without fluid to pressurize the system. Easier than pumping and no risk of a mess if the cap doesn't seal appropriately. Yes, it takes a bit longer to keep filling the reseviour which is a super PITA to fill due to the number of baffles in the thing. I do mine once a year. There has been no "color change" when moving the fluid out of the calipers which makes me wonder if I'm wasting my time to some degree. I track my car a fair amount and use SRF which as a wet boiling point higher than the dry on regular DOT4 by a fair margin. It's part of my yearly DCT fluid and filter change efforts before the season. Peace of mind mostly. I may do a moisture check next time to see where it's at.

 

I use a brake fluid tested and go from there. bleed when I change pads also, using a motive.

 

I use and will continue to use a motive pressure bleeder along with commanding the ABS brake bleed sequence with a scanner. Works flawlessly. I run Motul 600 so I change it at least once a season.

I’ve never unhooked my battery doing brake work, swap my pads all the time.

 

Why are vacuum pumps a no-no?

When I was racing, I would swap my brake fluid one or two times a year using a hand pump Mighty-Vac, connected to the bleeder on the caliper. Pull a vacuum, and continue adding fluid to the reservoir until the new fluid is seen exiting the caliper.

Is there something about the C8's brake system which makes this a problem?

 

I don't think the fluid ever cycles to reservoir like it used to do on clutch fluid of manual Corvettes? In other words, I don't think the fluid test at reservoir would help figure out the condition of fluid at the caliper? Am I missing something?

I have such a tester as well and used to use it on my manual clutch fluid back when I owned a manual C7 Z06, but haven't had much of a use on my C8s...

 

While the brake system doesn't displace fluid like a hydraulic clutch there is some movement and water would disperse through the system over time. The fluid in the reservoir would be representative of the system unless water was recently introduced somewhere else. If you really wanted to test it you could put some dye in the reservoir and check for dye at a brake bleeder after a month or so of driving.

 

In fact, if the fluid at the nether regions of the system (aka wheel cylinders) is contaminated with absorbed water, how did it get there? Through seepage around the cylinder seals, or did it enter at the reservoir and disperse through the fluid? (I agree that the the absorbed water will work it way through the the fluid even though the actual motion of the fluid during brake cycles is very, very small.)

 

I've seen contaminated and/or cooked fluid on the caliper end many times, while the reservoir fluid looked like new. Even if it does, the travel of fluid from the calipers to the reservoir seem to be very limited, at least in my experience.

 

Yes I agree - the travel is very small. Properly adjusted, the brake cylinder travel to engage the brakes is a fraction of an inch. I was commenting on dispersion of the absorbed water molecules, which I believe can happen without actual fluid flow - not other forms of contamination. The tester only tests for moisture content. I've seen the same thing with my older vehicles - I don't use the ranger method - I bleed them.

 

I don't know water gets in modern systems with a diaphragm on the reservoir. Nor can I remember having too much moisture when I test them (although I haven't tested many). My older vehicles, yes they get water in the system.

 

I agree , travel between the master cylinder and calipers is very small. There can be a bit of water intrusion. You have to view brake fluid or any hydraulic fluid as a solid "rod", snaking its way from the master and finally pushing up against the caliper pistons. In my experience, i agree with what was said. The fluid that takes the biggest beating is in the calipers, mainly from heat. Thats why the system should be flushed out. Personally, I like using a pressure bleeder.

 

Moisture can be introduced through condensation (especially with drastic heat cycles/deltas) and due to the hygroscopic nature of brake fluid, it is absorbed. This is so common and expected, brake fluid specs give you a "dry" and a "wet" boiling point. Wet is for when the moisture content is 2-3% or more.

When I was regularly road racing, I could tell when my brake fluid (Motul 600) needed flushing based on the brake fade I would start to get. The moisture in the fluid (at the caliper) would boil causing fade due to the vapor now in the line.

For a street driven vehicle, you will most likely never exceed the wet boiling point temperature though, and thus won't experience issues the same as a tracked vehicle when the moisture content is over 3%.

 

Yes, but where is the entrance point for air (or water) to enter the system and condense? Past the caliper seals? With a diaphragm on the reservoir it is basically a sealed system.

 

The concern isn't whether or not the system should be periodically flushed (it should), the concern was if you tested the reservoir for moisture that it wouldn't be representative of moisture in the calipers. Water is absorbed by the brake fluid, it isn't like contamination particles. If you put a drop of dye in a gallon of water it will eventually evenly disperse throughout the water without agitation. Similarly, absorbed water will eventually migrate throughout the brake system with little fluid movement. This little bit of movement is how discolored fluid reaches the reservoir.

 

Air enters through the air space in the reservoir, then condenses and the brake fluid absorbs the moisture. Once there is moisture in the fluid, it will travel and usually ends up concentrating at the calipers (low point in the system). Once it ends up in the caliper, it will boil if the temp exceeds the wet boiling point and create vapor. This is well accepted and proven, not sure why folks are questioning it?

A simple Google AI query produced the following:

Brake fluid is hygroscopic (absorbs water) and will gradually distribute moisture over a long period through diffusion, the moisture content is usually much higher in the calipers and wheel cylinders than in the master cylinder reservoir. [1, 2]

Key Facts About Moisture Distribution:

• Settling at Low Points: Moisture is heavier than brake fluid and tends to settle at the lowest points in the system, specifically in the brake calipers and wheel cylinders.
• Uneven Contamination: A test of the fluid in the reservoir may show low moisture content, while the fluid in the calipers could be heavily contaminated.
• Diffusion Over Time: While there is very little active circulation of fluid through the lines during normal driving, the moisture does migrate through the entire system via a process called diffusion over many months.
• Heat Increases Risk: The highest concentration of water is usually found in the calipers, which is also the hottest part of the system, making them prone to vapor lock. [1, 2, 3, 4]

Therefore, even if the brake fluid in the reservoir appears clear or tests clean, the fluid at the calipers can still contain enough moisture to cause rust or boil during heavy braking, which is why periodic flushing is necessary. [ , 6]

AI can make mistakes, so double-check responses
[1] https://phoenixsystems.co/blogs/company-news-tips/brake-fluid-moisture-testing-the-simple-check-that-explains-pedal-feel-corrosion-and-heat-fade
[2] https://www.irv2.com/threads/brake-f...level.2058231/
[3] https://www.reddit.com/r/AskMechanic...d_circulation/
[4] https://www.reddit.com/r/askscience/...d_hygroscopic/
[5] [6] https://bw-garage.com/brake-fluid-change-vs-flush/

 

As far as I know, most of the contamination in the caliper end of the fluid is not coming from reservoir, but rather from the pistons, especially after track events.

 

Check and compare the specific gravity of water and brake fluid. Water is 1 g/cm3. Brake fluid is (depending on where you look) 1.03-1.10 g/cm3. Isn't brake fluid slightly heavier?

Compare that with motor oil which will not absorb moisture so much, but rather will float on top of it. Water does sink in gasoline or motor oil.
The specific gravity of motor oil typically ranges from 0.84 to 0.91.

 

In either case, the moisture at the calipers is typically more than the reservoir ... here is what ChatGPT says about it:


Heat cycling makes it easier for existing moisture from outside the fluid to get in, and it can also concentrate the effects of that moisture at the hot ends of the system.

What heat cycling does:

When the caliper area gets hot and then cools repeatedly, several things happen:

1. Rubber parts breathe a little.
Flexible hoses and seals are not perfectly impermeable. Repeated expansion and contraction, along with temperature-driven pressure changes, can slightly increase vapor permeation over long periods.

2. Diffusion happens faster when hot.
Higher temperature generally increases the rate at which water vapor can migrate through elastomers and into the brake fluid.

3. Cooling can create small pressure changes.
As components cool, internal pressure can drop relative to ambient conditions. The system is not open like a vented tank at the caliper, but these pressure swings can encourage tiny amounts of moisture ingress past microscopic permeation paths over time.

4. Hot fluid degrades faster.
Even if the water amount did not increase much, heated fluid at the caliper becomes chemically “older” faster, so that end of the system tends to become the worst part first.

So the key distinction is:

• Heat cycling does not make water
• Heat cycling accelerates moisture entry and fluid degradation

A good analogy is a plastic food container in a humid room: heating and cooling the container does not create water, but it can make moisture transfer through the material happen faster and make condensation effects more noticeable when temperatures swing.

In a brake system, the original moisture sources are still things like:

• humidity exposure at the reservoir
• slow permeation through rubber hoses and seals
• service work or fluid replacement done in humid air

Heat cycling just helps those outside moisture sources have more effect at the wheel ends.