You can only try, sometimes people will not believe,

"As the pads & discs wear, the pistons move further out. The fluid level in the reservoir drops. There is not a vacuum in that reservoir.

So, air has entered. It's not sealed.

Where am I wrong?"

 

Conversely, I've never seen significant water or contamination in a properly functioning brake system in a car, other than some 30-50 year old toys I get to play with that have been neglected. Leaky calipers that corroded and seized yes,- but that was mechanical failure of the seals as root cause, not absorption of water. DOT5 would not have made a difference. The C2/C3 Corvette guys have been battling seal failure exacerbated by rotor runout for decades. I've got mine working perfectly and consistently and am reluctant to disturb it.

Swapping to 5.1 would a decent way of lightening a wallet, but otherwise what would be proved? The DOT4 in our S-types never comes anywhere near the boiling point, so an increased level would have no purpose.

I have a tough time believing the idea that water is absorbed through the walls of a plastic container. Why would it not then be absorbed through the walls of the plastic master cylinder reservoir? Can you point me to any credible tests or other evidence?

From October of this year:

Most vehicles have had a diaphragm separating the air from fluid since the '60s. I think the carved in stone recommendation to change fluid every two years dates back to the bad old days before that- much like the much revered 3000 mile oil change interval.

 

OK, so 3 questions:
1. why does the fluid in the reservoir change colour bit by bit?
2. why do racers find they do need to bleed fresh fluid through, as Bob says?
3. why do manufacturers still say to change the fluid so often? It's against their interests for such as fleet buyers who want the absolute minimum of servicing (preferably none - which they don't get - over at least the first 3 years).

For #1 I suggest something like the seal is less than perfect.

 

Dunno.

Why do birds suddenly appear every time........
Why do dogs lick them..............
Why do people not read what's already posted before making snide comments like 'You can only try, sometimes people will not believe, '

and a thousand more.

Let's say the seal at the M/C is not perfect and moisture does contaminate the fluid. This would take a considerable amount of time to become significant, certainly not instantly. The brake system is not a recirculating type so the contaminated fluid would simply sit in the M/C and never make it's way down to the wheel cylinders where it could possibly boil.

So why do the racing guys need to change fluid so often? Dunno.

 

And #3?

 

Can't offer much more than 'dunno' once again. Maybe a pancake bunny?


I do know that the vast majority of owners (all brands, not just Jags) never change the brake fluid, ever. The days of twice annual 'tune-ups' where fluid levels were checked or a replacement opportunity arose are long gone. Most people wouldn't know where to find the M/C reservoir, never mind the trouble of going to the FLAPS to buy fluid to do a top up. The concept of scheduling a visit to a garage on a two year schedule simply to change brake fluid is not on many people's radar, so it doesn't get done on probably 99% of cars.

Yet- cases of brake failure due to fluid contamination are virtually unheard of.

 

I know brake fluid change is part of BMW's service schedule, and nowadays part of condition based service (the car reminds you what needs to be done and when). Hard to ignore...

 

Again ALL cars absorb water into the brake fluid. Easy to tell. The color changes from clear to yellow to brown. The brakes just degrade over time. Even sludgy fluid will work to some extent. It really more about getting the most from your brake system. Regular bleeding and flushing keeps the brake pedal feel much better. Also how long do you keep cars? If only a short time you can ignore maintenance. Since I do my own wrenching and am cheap I find it's good insurance for keeping the M/C and calipers sludge free.

I guess the people who make and sell high performance brake products are wrong?? Performance Friction Brakes has a nice page on this. Read about the plastic container issue on their brake fluid page. Very informative.
performancefrictionbrake

Other brake manufactures have similar information.
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You've yet to explain how it gets there. How do you know that it changes colour due to moisture and not some other factor? If you boil it, does it go back to being clear?

Could you explain the physics of why (supposedly) contaminated fluid changes the feel of a brake pedal?



Your link doesn't work, so I looked up the mfgr. Here's picture of their brake fluid- in plastic bottles (?)



Performance Friction - Brake Fluid.


Aside from the Jag and her Honda, the youngest vehicle in our fleet (with hydraulic brakes) is an '81. The oldest is a '68. We keep them all a very long time.

 

How would I find out statistics on accidents attributed to brake issues? Or any issues? Are they even investigated? AFAIK here any such figures would not be added up or published.

Over here most people have their cars serviced by a garage (I think you say "shop") and they'll change the fluid as per the schedule. I suppose that would mean very low figures of accidents due to brake issues, here at least.

If the schedules are wrong then it's odd none of the consumer focus groups appear to be saying so, but maybe they'll be doing so soon.

 

If the fluid starts boiling, it is not a hydraulic system anymore. Liquid is not compressable whereas gas is.

 

We're going in circles...............

 

Sorry for the bad link!! See if this works.

Performance Friction Brakes
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When I started maintaining my car and noticed that the brake liquid was brown, it was the same color in calipers too. So clearly it does mix all around.

 

Now a different circle.

The fact that it was brown throughout indicates that possibly:

1) it was brown to start with

2) it turns brown due to a factor other than moisture. Maybe exposure to rubber seals?

 

Never seen new brake fluid like that. Possible, of course, but highly unlikely. I haven't seen every single brake fluid on the market last ten years.

Following your hypothesis that the fluid just moves back and forth, shouldn't it then be brown only where the rubber seals are and not consistently brown.

Where do you get the idea that the fluid is motionless in the first place? The temperature is motion of molecules and use of brakes moves the fluid in brake lines which are not perfectly flat inside. I am sure there are other reasons too.

 

There's rubber seals in the M/C and each caliper. You've noticed brown fluid coming out of the caliper, others have noticed it the M/C.

Heat causes movement at the molecular level, resulting in expansion. Convective currents (presuming there is a temperature differential) may cause circulation within a body of liquid. Proposing that this flow( or any conductive heat) is sufficient to reach the M/C is hard to grasp.

As for flow for other reasons, how about somebody pour some dye in their M/C then drive the car to see how long it takes for this coloured fluid to appear in the calipers.

 

As the fluid is so keen to grab moisture, it will slowly disperse that moisture fairly evenly throughout the fluid. Somewhere there's probably a measure indicating how fast that occurs, but I don't know where to look!

 

Right the color change is from the moisture causing corrosion. The fluid tends towards brown because of the steel that is rusting. As the brake systems get more and more Aluminum in them the color is less noticeable because Aluminum corrodes less and the byproducts are of a lighter color. I see the brake fluid these days turning more yellow than brown.
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This post is becoming a bit long but really informative. I agree that changing the fluid every 2 years in a street driven car is certainly overkill. Dealers make most of their profit on servicing and this is probably just another gaff to lighten our wallets.
Race cars on the other hand require bleeding before or after every track event. The temps run so high that the fluid virtually cooks in the calipers. Rotors usually crack from the heat long before they wear out. (hey, maybe another thread: Drilled vs. plain or slotted rotors) Although "ATE Super Blue" is pretty good stuff and practical because of the color change, I ve seen it boil at the track. Motul 600 seems to work really well. There's other great products, eg. Castrol, but a lot more expensive.