Any - Bubble Bubble, Boiling Brake Fluid Trouble. | Technical matters | Back Room Forum

Fri 3 Jul 2015 12:01

Any - Bubble Bubble, Boiling Brake Fluid Trouble. - edlithgow

Been having some discussion/argument elsewhere on this.

www.forumosa.com/taiwan/viewtopic.php?f=75&t=1...1

Current issue is a bit of a 'tis/t'isnt on whether brake fluid boiling in, say the rear brakes is likely to kill the master cylinder (I assume by transferred heat and/or pressure).

I say it isn't likely, but I don't know this for a fact, and may be suffering from wishful thinking.

Here's the most relevant bit of the thread, snipped from some corner of a foreign website.

The HJ forums have a bigger "knowledge base" and there's probably more experience and/or educated opinion available here.

"Alex1000 wrote:

IMHO. If boiling is started somewhere (assume in a rear wheel) soon it will happen everywhere in whole breaking system."

Can't see that. It'll boil locally, where the heat is. As the steam expands up the brake pipes, it'll cool and re-condense. If you kept driving and feeding more heat into the system eventually you'd heat it all up and I suppose the steam would travel the long, long, narrow, heat dissipating metal pipe to the master cylinder, but meanwhile, you've had a brake failure, and either:-

(a) scared the s*** of yourself and stopped.

(b) scared the s*** of yourself and crashed.

And stopped.

Either way you ain't cookin' no master cylinder no more, no how.

EDIT: I suppose sustained driving without using the brakes could get you to this situation, but I'd think that isn't very likely to happen (even on-freeway) in Taiwan. ENDEDIT

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Fri 3 Jul 2015 12:50

Any - Bubble Bubble, Boiling Brake Fluid Trouble. - Bolt

contrary to what was posted, is, possible for master cylinder to fail, you can get soft seals which turn back on themselves when sudden pressure is applied to the pedal, causing brake failure

A bit surprised some have said it cant happen

All a duel circuit is and can be a diagonal system or front rear seperate system, if a wheel cylinder fails on one wheel losing brake on that circuit the other will work, if the master cylinder fails all brakes fail which seems to have occured here imo

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Fri 3 Jul 2015 13:12

Any - Bubble Bubble, Boiling Brake Fluid Trouble. - edlithgow

Thanks for your reply.

I agree that there's no doubt that master cylinder seals can fail.

The issue here is, IF brake fluid in a rear wheel has boiled (which seems likely in at least one of the cases under discussion) is it likely to damage the master cylinder?

I guess (and argue) probably not, but I don't know.

As a slightly separate issue, your mention of master cylinder seals "turning back on themselves" makes me wonder if that could be a reversible/intermittent fault that could "spontaneously" recover.

If it could, that weakens the strongest argument (spontaneous recovery) for boiling brake fluid as the most likely cause of these incidents.

Edited by edlithgow on 03/07/2015 at 13:22

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Fri 3 Jul 2015 13:50

Any - Bubble Bubble, Boiling Brake Fluid Trouble. - Bolt

It usually is intermitent, and can happen at any time, as for boiling brake fluid reaching the master cylinder I would say impossible due to air flow cooling the pipe between hub and MC

the only issue would be losing the brakes on that circuit due to gas compressing, though you normally suffer brake fade before that occurs due to pads/shoes overheating and losing friction

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Fri 3 Jul 2015 14:48

Any - Bubble Bubble, Boiling Brake Fluid Trouble. - hardway

Brake fluid does not circulate.

Presure applied at the pedal pressurises the line.

Fluid in the rear cylinder STAYS in the cylinder.

Even ABS the fluid from the ABS pump down the lines to callipers/slaves doesn't move as such.

and does not curculate back to the master.

So heat transfer would be via the brake hard line all the way back to the master.

Difficult to see that happening in what amounts to an air cooled pipe,

Even the type covered with a shield like the S Type up in the air on my ramp just now.

I was taught that it is the moisture content of the fluid that causes problem,

over heatin cuases the fluid to "boil" the water into vapour which unlike the fluid itself IS compressable so your foot goes all the way to the floor!

I even had my local Snap On guy in last month or so trying to sell me their latest fluid hygrometer tester.

But I'll stick with my practice,

I renew a hard line I usually bleed/flush the entire system.

I don't like takeing chances with brakes!

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Fri 3 Jul 2015 16:57

Any - Bubble Bubble, Boiling Brake Fluid Trouble. - Andrew-T

I was taught that it is the moisture content of the fluid that causes problem, over heating causes the fluid to "boil" the water into vapour which unlike the fluid itself IS compressable so your foot goes all the way to the floor!

Yes. 'Boiling' happens when any water/moisture in the fluid reaches 100°C or more. Clean brake fluid boils at about 300° which is seriously hot. So if the fluid has been changed according to schedule it shouldn't happen in normal circumstances.

Standard fluid tends to absorb moisture over time, which is why some prefer to use silicone fluid, which is incompatible with water.

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Fri 3 Jul 2015 17:32

Any - Bubble Bubble, Boiling Brake Fluid Trouble. - bathtub tom

If someone managed to boil the brake fluid, you wonder what their maintenance regime is like, or could be ignoring a problem in the hope it goes away.

A problem with the master cylinder could be just another result of lack of care.

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Sat 4 Jul 2015 09:33

Any - Bubble Bubble, Boiling Brake Fluid Trouble. - edlithgow

Can boiling brake fluid defeat a dual circuit system, so there is no residual braking?

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Sat 4 Jul 2015 09:55

Any - Bubble Bubble, Boiling Brake Fluid Trouble. - Bolt

Can boiling brake fluid defeat a dual circuit system, so there is no residual braking?

IF, and I mean If, it happened, then you would lose all braking but its unlikely. dual circuit is controlled by the master cylinder

www.carbibles.com%25252Fbrake_bible_pg2.html&s...D" target="_blank">https://www.google.co.uk/search?q=dual+circuit+brake+master+cylinder&biw=1280&bih=683&tbm=isch&imgil=4mrkGwtYxRDJOM%253A%253BeflkFzjLTVRpcM%253Bhttp%25253A%25252F%25252Fwww.carbibles.com%25252Fbrake_bible_pg2.html&s...D

No, you are more likely to suffer brake fade than boil the fluid.

brake fade happens a fair bit even these days if pads and discs are extremely worn, as heat dissipation is slower

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Sat 4 Jul 2015 09:47

Any - Bubble Bubble, Boiling Brake Fluid Trouble. - edlithgow

If someone managed to boil the brake fluid, you wonder what their maintenance regime is like.

In my case neglect was certainly a factor, with the fluid long-overdue for a change.

The car that's the subject of the linked thread (which goes on a bit) is a dealer serviced Mazda, but there's some doubt as to whether a brake fluid change is on the service schedule.

One of the linked sources states that Ford consider brake fluid to be a non-service item. (!)

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Sat 4 Jul 2015 12:09

Any - Bubble Bubble, Boiling Brake Fluid Trouble. - Bolt

imo, no matter what the car, I would change master cylinder and fluid which would be automatic anyway after a MC change

I`m inclined to think it was MC failure anyway.

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Mon 6 Jul 2015 11:01

Any - Bubble Bubble, Boiling Brake Fluid Trouble. - jc2

If someone managed to boil the brake fluid, you wonder what their maintenance regime is like.

In my case neglect was certainly a factor, with the fluid long-overdue for a change.

The car that's the subject of the linked thread (which goes on a bit) is a dealer serviced Mazda, but there's some doubt as to whether a brake fluid change is on the service schedule.

One of the linked sources states that Ford consider brake fluid to be a non-service item. (!)

If you look in Ford ETIS,you will find that Ford recommend a fluid change every two years.

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Wed 8 Jul 2015 11:53

Any - Bubble Bubble, Boiling Brake Fluid Trouble. - skidpan

so I wasn't fully aware of the need to replace it anyway on a precautionary basis

Our last cars have been Fords, Nissans, a Seat, a BMW and a Kia and all have required a brake fluid service at 2 years. Not expensive when you consider the consequences of brake failure.

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Wed 8 Jul 2015 11:54

Any - Bubble Bubble, Boiling Brake Fluid Trouble. - edlithgow

If someone managed to boil the brake fluid, you wonder what their maintenance regime is like.

In my case neglect was certainly a factor, with the fluid long-overdue for a change.

The car that's the subject of the linked thread (which goes on a bit) is a dealer serviced Mazda, but there's some doubt as to whether a brake fluid change is on the service schedule.

One of the linked sources states that Ford consider brake fluid to be a non-service item. (!)

If you look in Ford ETIS,you will find that Ford recommend a fluid change every two years.

So it does (I looked up a 1989 Fiesta since my last car, a Ford Sierra DOHC, wasn't listed. Maybe too few left?).

Could be the source mentioned above is out of date, its likely to be specific to the US (I THINK ETIS is for Europe) or it could just be wrong.

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