Nagging brake fluid curiosities

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Context: newer cars, 2010 and newer.

A. I know that brake fluid isn't in a 'circulating' system like coolant but.... Does it move in the lines very far at all?
B. do we basically brake upon (heat/degrade/abuse) the same fluid contained at the last perhaps 5 inches of line/caliper for the entire time that fluid is in the car?!
C. If by some weird chance one brake doesn't get bled , is there danger? And will the contaminants in that line migrate into the whole system?
D. Related note: does brake fluid exhibit osmotic type flow of contaminents even if held still. Example : if I had an experimental brake line on my bench with a funnel at each end , and I add new fluid at one end and old at the other simultaneously and they met in the middle being poured in.... Would contaminents such as moisture be "drawn into " the new fluid over a few weeks months?
E. Does ABS activation cause any significant system wide flow or exchange of fluid between brake lines?
 
Originally Posted By: SumpChump
Context: newer cars, 2010 and newer.

A. I know that brake fluid isn't in a 'circulating' system like coolant but.... Does it move in the lines very far at all?

Based on my observations the fuid moves merely a few inches as evidenced by dark fluid being quickly followed by clear fluid when bleeding brakes.

B. do we basically brake upon (heat/degrade/abuse) the same fluid contained at the last perhaps 5 inches of line/caliper for the entire time that fluid is in the car?!

I think so.


C. If by some weird chance one brake doesn't get bled , is there danger? And will the contaminants in that line migrate into the whole system?

Three out of four ain't bad.
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D. Related note: does brake fluid exhibit osmotic type flow of contaminents even if held still. Example : if I had an experimental brake line on my bench with a funnel at each end , and I add new fluid at one end and old at the other simultaneously and they met in the middle being poured in.... Would contaminents such as moisture be "drawn into " the new fluid over a few weeks months?

Probably to a degree. I think the heavier contaminates will stay mostly in and near the caliper. I don't know about the water content, but I suspect it will spread throughout the system until equilibrium is reached.


E. Does ABS activation cause any significant system wide flow or exchange of fluid between brake lines?

I doubt it.
 
Great topic and questions and useful to a lot of people who don't think enough about brakes on multiple cars (that's me all over). I keep thinking I "ought" to change it and one is very overdue. Thank you.
 
I think there's circulation of brake fluid due to the fact that when ABS is actuated brake fluid pressure is relieved from the locked wheel (that fluid has to go somewhere) AND the ABS system pump kicks in to maintain pedal feel and height.

All this takes place in the ABS controller block. The circulation may be tiny or may involve dumping the relieved fluid into a reservoir.

The circulation might all take place at the controller block and the stale fluid in the slave cylinder and the preceding 2 feet of brake line could be as the OP described. I'm not certain.

Is it not the case that actuating the ABS pump is part of updated brake bleeding? Kira
 
One possible thing to remember. When this comes up people tend to poo-poo it a little - oh it's easy to do....drain, add, pump, bleed, etc. And it is but if this hasn't been done for a long time the master cylinder can have a built up area at the limit of piston travel. Pumping and bleeding causes that piston seal to rub/drag over that ridge possibly damaging it. Anyhow, there is a need to pay attention after a bleeding. A strange feeling pedal might not be from bleeding that didn't go well.
 
Brake fluid pressure goes right through the ABS solenoid(8) block during normal braking as if the ABS unit wasn't there.I think KIRA is right about fluid circulation but there is no "reservoir" on my 04 ABS system. The first Honda I purchased did have a surge chamber of sorts but the bleeding process was the same. I seem to remember that some Honda ABS systems had a nitrogen charged accumulator that required special equipment the bleed the system. By 1992 that accumulator had been deleted from the ABS system.
 
The brake fluid goes back and forth by the volume of the master cyl. Think of the master cylinder and slave cylinder being like a like a hypodermic syringe each connected by a hose. The Brake fluid is pushed one way then the other. ABS brakes the valves release and apply .
 
Originally Posted By: DeepFriar
One possible thing to remember. When this comes up people tend to poo-poo it a little - oh it's easy to do....drain, add, pump, bleed, etc. And it is but if this hasn't been done for a long time the master cylinder can have a built up area at the limit of piston travel. Pumping and bleeding causes that piston seal to rub/drag over that ridge possibly damaging it. Anyhow, there is a need to pay attention after a bleeding. A strange feeling pedal might not be from bleeding that didn't go well.


If you are worried this will happen, the car will need to be pressure bled, vacuum bled, or gravity bled, because any pedal movement, no matter how small the movement, with a line open will cause the master cylinder piston to move a distance that it never does during normal use.

My worthless and unsolicited opinion: If pumping the pedal during bleeding ruins the master cylinder, it was already unsafe junk waiting to fail. Only on museum cars that are never driven should these precautions be taken. I think it is better if you root out a soon to fail master cylinder while you are alert during or shortly after a brake service, rather than later when complacency resumes.

Great point on driving with extreme care after the brakes are bled, or after any brake work for that matter. It is extremely easy for air to accidently get in the lines, even with the best of equipment and personnel.

If the brakes are flushed and bled every two to three years, there shouldn't be enough build-ups or deposits to worry about.
 
So the consensus seems that:

We do indeed brake upon the fluid in the caliper from FACTORY fill which includes just a few precious ounces of fluid that take the real abuse and does the real stopping. (Until and unless bled)

There is no real circulation of fluid and the lines and abs and Resevoir ante all basically stale for the service life of that particular fluid fill.

Fluid WILL have an osmotic dissipation of contaminents if new flui collides with old such as via a Resevoir drain and fill. However the time and effect are totally unknown. It would take an experiment to figure that out.

FOOD FOR THOUGHT (which I probably should make a separate posting)
Doing a Resevoir drain and fill may actually be counterproductive. This because it may "accidentally fool" (not maliciously deceive) a mechanic or other family member into thinking the fluid system wide is clean and viable. Also the Resevoir drain and fill of perhaps a year ago may make it difficult to do the classic "see when clean fluid comes out the bleeder". No distinguishing visual properties to assisted the person doing the service. I may indeed wait a few days and post that as a separate thread.
 
If the brakes are flushed and bled every two to three years, there shouldn't be enough build-ups or deposits to worry about. [/quote]

Good points and I agree. There are a lot of "normal" people who don't think of this for longer than that though. It was for them I was concerned. There is some hideous fluid out there.
 
Originally Posted By: SumpChump

D. Related note: does brake fluid exhibit osmotic type flow of contaminents even if held still.


Yes, it's called diffusion, not osmosis.
In any liquid (as well as gas) above absolute zero temperature the molecules are moving around and bouncing off each other in random directions.
Some particles settle down due to gravity and (the lack of) buoyancy, but an individual molecule (say, or water) can travel a long distance if given enough time.
 
Originally Posted By: Nate1979
So why does the fluid in the reservoir become darker over time?


Contaminated fluid at the calipers can diffuse all the way to the reservoir.
It can take years, but it happens.
 
I agree on that if you bleed brakes every 2 or 3 years, you will not see all these problems, I also heard about bleeding the conventional 2 person way is bad for the MC travel and could have potential problems, well, in my experience, the 5 cars I service and bleed brakes every 2 years religiously didn't have the potential problem mentioned, in fact, the brake feel is good as when brand new when you use a good brake fluid like ATE, or OEM fluid in my experience.
 
Originally Posted By: circuitsmith
Originally Posted By: Nate1979
So why does the fluid in the reservoir become darker over time?


Contaminated fluid at the calipers can diffuse all the way to the reservoir.
It can take years, but it happens.


I thought the Resevoir became dark because most or many are vented to atmosphere.
 
They are not vented to the atmosphere, there is a rubber diaphragm under the master cylinder cap preventing direct contact between the fluid and outside air.
 
I like to keep a little grease on the bleeder so it doesn't get frozen-and do a little gravity bleeding from time to time.
 
Originally Posted By: SumpChump
I thought the Resevoir became dark because most or many are vented to atmosphere.


Here's an experiment:
Put a little brake fluid in an open container, like a bottle cap.
Sit it on a shelf and see if it darkens.
 
Originally Posted By: circuitsmith
Originally Posted By: Nate1979
So why does the fluid in the reservoir become darker over time?


Contaminated fluid at the calipers can diffuse all the way to the reservoir.
It can take years, but it happens.


But it will not work the other way around (i.e.; clean, new fluid put into the reservoir diffusing to the calipers even after years and years)?
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