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From: Chris Whelan on 29 Jun 2010 16:26
On Tue, 29 Jun 2010 20:36:04 +0100, Paul wrote:

[...]

> And the stuff in the reservoir in theory never gets used, as its there
> to replace any lost by a leak...

It's there primarily to account for the change in volume caused by the
friction materials wearing. For example. as the pads wear, the caliper
pistons move further out, and more fluid is needed to make up that space.

The water is heavier than the brake fluid, so eventually finds it way to
the lowest part of the system, which in most conventional cars is the
rear wheel cylinders.

Guess which part of the braking system is usually the first to succumb to
corrosion damage :-)

Chris

--
Remove prejudice to reply.
From: Chris Bartram on 29 Jun 2010 16:27
On 29/06/10 18:12, Tim Downie wrote:
> My dealer would like to change my VW touran brake fluid every two years.
> Is this being over cautious or sensible preventative maintenance?
>
> Tim
Sensible, and the cost is not high.
From: Bob Dodds on 29 Jun 2010 17:45
On 29 June, 21:26, Chris Whelan <cawhe...(a)prejudicentlworld.com>
wrote:
> On Tue, 29 Jun 2010 20:36:04 +0100, Paul wrote:
>
> [...]
>
> > And the stuff in the reservoir in theory never gets used, as its there
> > to replace any lost by a leak...
>
> It's there primarily to account for the change in volume caused by the
> friction materials wearing. For example. as the pads wear, the caliper
> pistons move further out, and more fluid is needed to make up that space.
>
> The water is heavier than the brake fluid, so eventually finds it way to
> the lowest part of the system, which in most conventional cars is the
> rear wheel cylinders.
>
> Guess which part of the braking system is usually the first to succumb to
> corrosion damage :-)
>
> Chris
>
> --
> Remove prejudice to reply.

OK if all this is true and this much moisture is added to the oil how
come I've never had to remove oil from a hydraulic reservoir to make
up for the water added? There must be a lot of water in the oil to
become enough to cause all this corrosion.

Bob
From: Duncan Wood on 29 Jun 2010 17:51
On Tue, 29 Jun 2010 22:45:47 +0100, Bob Dodds <13013dodds(a)gmail.com> wrote:

> On 29 June, 21:26, Chris Whelan <cawhe...(a)prejudicentlworld.com>
> wrote:
>> On Tue, 29 Jun 2010 20:36:04 +0100, Paul wrote:
>>
>> [...]
>>
>> > And the stuff in the reservoir in theory never gets used, as its there
>> > to replace any lost by a leak...
>>
>> It's there primarily to account for the change in volume caused by the
>> friction materials wearing. For example. as the pads wear, the caliper
>> pistons move further out, and more fluid is needed to make up that
>> space.
>>
>> The water is heavier than the brake fluid, so eventually finds it way to
>> the lowest part of the system, which in most conventional cars is the
>> rear wheel cylinders.
>>
>> Guess which part of the braking system is usually the first to succumb
>> to
>> corrosion damage :-)
>>
>> Chris
>>
>> --
>> Remove prejudice to reply.
>
> OK if all this is true and this much moisture is added to the oil how
> come I've never had to remove oil from a hydraulic reservoir to make
> up for the water added? There must be a lot of water in the oil to
> become enough to cause all this corrosion.
>
> Bob

They're miscible & it doesn't take very much.

--
Duncan Wood
From: Bob Dodds on 29 Jun 2010 18:02
On 29 June, 22:51, "Duncan Wood" <nntp...(a)dmx512.co.uk> wrote:
> On Tue, 29 Jun 2010 22:45:47 +0100, Bob Dodds <13013do...(a)gmail.com> wrote:
> > On 29 June, 21:26, Chris Whelan <cawhe...(a)prejudicentlworld.com>
> > wrote:
> >> On Tue, 29 Jun 2010 20:36:04 +0100, Paul wrote:
>
> >> [...]
>
> >> > And the stuff in the reservoir in theory never gets used, as its there
> >> > to replace any lost by a leak...
>
> >> It's there primarily to account for the change in volume caused by the
> >> friction materials wearing. For example. as the pads wear, the caliper
> >> pistons move further out, and more fluid is needed to make up that  
> >> space.
>
> >> The water is heavier than the brake fluid, so eventually finds it way to
> >> the lowest part of the system, which in most conventional cars is the
> >> rear wheel cylinders.
>
> >> Guess which part of the braking system is usually the first to succumb  
> >> to
> >> corrosion damage :-)
>
> >> Chris
>
> >> --
> >> Remove prejudice to reply.
>
> > OK if all this is true and this much moisture is added to the oil how
> > come I've never had to remove oil from a hydraulic reservoir to make
> > up for the water added? There must be a lot of water in the oil to
> > become enough to cause all this corrosion.
>
> > Bob
>
> They're miscible & it doesn't take very much.
>
> --
> Duncan Wood- Hide quoted text -
>
> - Show quoted text -

That makes it more puzzling by being miscible you would need a lot
more moisture to contaminate the oil. If they stayed separated and as
said previously the water migrated to the lowest point you would need
less contamination to affect the caliper pistons.

Bob
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