How much does brake fluid circulate around the braking system?

The Revd

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Nov 25, 2011, 9:19:30 AM11/25/11

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Does it all basically stay where it is, or does it circulate?

Hence, is the occasional syphoning out of brake fluid from the
reservoir and replacing it with fresh fluid worthwhile?

Chris Whelan

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Nov 25, 2011, 11:20:17 AM11/25/11

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On Fri, 25 Nov 2011 09:19:30 -0500, The Revd wrote:

> Does it all basically stay where it is, or does it circulate?

IME, stays where it is.

> Hence, is the occasional syphoning out of brake fluid from the reservoir
> and replacing it with fresh fluid worthwhile?

No. The primary contaminant is water adsorbed from the atmosphere. This
is heavier than the fluid, so sinks to the lowest point. This is usually
the rear wheel cylinders, and by a cruel twist of fate (!), these are the
components most at risk from corrosion damage. You can see this if you
bleed the rear brakes; it's only the first few pushes of the brake pedal
that produces dirty fluid.

Chris

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Remove prejudice to reply.

Rob

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Nov 26, 2011, 6:10:35 AM11/26/11

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Water actually gets in from both ends - the M/Cyl and w/cyl - you will
find that the m/cyl will also corrode. calipers will rust up as well but
not as accelerated as the w/cyl. To keep your system in good order fluid
should be changed (flushed out) on a yearly basis - probably something
that is far too ofter neglected.

Dave Plowman (News)

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Nov 26, 2011, 6:29:15 AM11/26/11

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In article <4ed0c939$1...@dnews.tpgi.com.au>,

Rob <mesa...@gmail.com> wrote:
> Water actually gets in from both ends - the M/Cyl and w/cyl - you will
> find that the m/cyl will also corrode. calipers will rust up as well but
> not as accelerated as the w/cyl. To keep your system in good order fluid
> should be changed (flushed out) on a yearly basis - probably something
> that is far too ofter neglected.

Even BMW only recommended a 2 yearly change.

--
*The last thing I want to do is hurt you. But it's still on my list.

Dave Plowman da...@davenoise.co.uk London SW
To e-mail, change noise into sound.

Chris Whelan

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Nov 26, 2011, 6:45:56 AM11/26/11

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On Sat, 26 Nov 2011 22:10:35 +1100, Rob wrote:

[...]

> Water actually gets in from both ends - the M/Cyl and w/cyl - you will
> find that the m/cyl will also corrode. calipers will rust up as well but
> not as accelerated as the w/cyl. To keep your system in good order fluid
> should be changed (flushed out) on a yearly basis - probably something
> that is far too ofter neglected.

I'd be interested to understand how water enters the system from the
wheel cylinders, but fluid doesn't get out.

Even Ford only recommend a 2 yearly change. ;-)

Rob

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Nov 26, 2011, 8:27:21 AM11/26/11

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Two years now may be correct but it was less, may have something to do
with the properties of the fluid or spare part sales. The MGB manual
says every 18 mths or 24000mls which ever comes first. I do mine every
12 mths and it stays in good nick, does about 3000mls/year

makes you wonder! how it gets passed the one way seals, I think its the
viscosity difference.

Its something less than a couple of percent water is the max in fluid.

In the good old days when they did good bogey jobs I remember brake
fluid being boiled to remove the water and then reused. But wheel
cylinders were made of better material and just a hone and new seals
fixed the brakes. Now its an assembly replacement.

They did more maintenance back then when you could maintain your own car.

rp

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Nov 26, 2011, 8:49:16 AM11/26/11

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On Sat, 26 Nov 2011 22:10:35 +1100, Rob wrote:

>Water actually gets in from both ends - the M/Cyl and w/cyl - you will
>find that the m/cyl will also corrode. calipers will rust up as well but
>not as accelerated as the w/cyl.

I thought the moisture only gets in from the reservoir as it's level
goes up and down as you brake and the vent in the reservoir cap allows
air in and out. The water is heavier than the fluid and sinks to the
lowest point, the wheel cylinders, and rusts them that way.

--
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The from address exists but is mostly dumped,
please send any emails to the address below
e-mail rpont (at) gmail (dot) com

Newshound

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Nov 26, 2011, 9:04:46 AM11/26/11

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On 26/11/2011 11:45, Chris Whelan wrote:
> On Sat, 26 Nov 2011 22:10:35 +1100, Rob wrote:
>
> [...]
>
>> Water actually gets in from both ends - the M/Cyl and w/cyl - you will
>> find that the m/cyl will also corrode. calipers will rust up as well but
>> not as accelerated as the w/cyl. To keep your system in good order fluid
>> should be changed (flushed out) on a yearly basis - probably something
>> that is far too ofter neglected.
>
> I'd be interested to understand how water enters the system from the
> wheel cylinders, but fluid doesn't get out.
>

Diffusion through the rubber seals. The water molecule is much smaller
than they hydraulic fluid one.

Chris Whelan

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Nov 26, 2011, 9:41:57 AM11/26/11

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On Sat, 26 Nov 2011 14:04:46 +0000, Newshound wrote:

[...]

> Diffusion through the rubber seals. The water molecule is much smaller
> than they hydraulic fluid one.

The seals are protected by rubber boots, are kept mostly dry and
sometimes warm, and the system pressure WRT atmosphere is either positive
or neutral.

I'd take a lot of convincing that more than a trivial amount of water
entered the braking system via the wheel cylinders!

Harry Bloomfield

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Nov 26, 2011, 1:22:06 PM11/26/11

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Chris Whelan was thinking very hard :

> I'd take a lot of convincing that more than a trivial amount of water
> entered the braking system via the wheel cylinders!
>
> Chris

Me too!

So far as I am concerned, the only mechanism for absorbing moisture in
the fluid, is from the air/fluid interface in the master cylinder. Then
I suppose that slightly heavier that pure fluid, fluid containing some
moisture might make its way eventually down to the wheel cylinders.

--
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Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk

dr6092

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Nov 26, 2011, 6:09:57 PM11/26/11

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On Nov 26, 11:45 am, Chris Whelan <cawhe...@prejudicentlworld.com>
wrote:

> I'd be interested to understand how water enters the system from the
> wheel cylinders, but fluid doesn't get out.

Osmosis through the flexible pipes.

Biggles

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Nov 26, 2011, 8:58:05 PM11/26/11

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On 26/11/2011 13:49, rp wrote:
> On Sat, 26 Nov 2011 22:10:35 +1100, Rob wrote:
>
>> Water actually gets in from both ends - the M/Cyl and w/cyl - you will
>> find that the m/cyl will also corrode. calipers will rust up as well but
>> not as accelerated as the w/cyl.
>
> I thought the moisture only gets in from the reservoir as it's level
> goes up and down as you brake and the vent in the reservoir cap allows
> air in and out. The water is heavier than the fluid and sinks to the
> lowest point, the wheel cylinders, and rusts them that way.
>

I don't pretend to know how water permeates into the hydraulic system
but the water contaminated fluid from the master cylinder would surely
have to go back uphill (from the underside of the floor) to get to the
rear wheel cylinders/calipers (which are always mounted towards the top
of the axle)?

Biggles

Steve Firth

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Nov 27, 2011, 2:57:33 AM11/27/11

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Bullshit.

Duncan Wood

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Nov 27, 2011, 6:28:55 AM11/27/11

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I don't think the water needs to accumulate there, it's there that's made
of cast iron so it'll discolour the fluid much sooner.

Newshound

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Dec 4, 2011, 6:40:16 PM12/4/11

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Perhaps it does all get in at the master cylinder, and gravity takes the
denser wet fluid to the "low point", but then diffusion will take water
uphill to the rear slaves. One of those things which gets more
complicated when you look at it closer.

Certainly true that in neglected vehicles it's the rear slaves which
suffer. I should know.