Brake fluid change

My dealer would like to change my VW touran brake fluid every two years. Is this being over cautious or sensible preventative maintenance?

Tim

Reply to
Tim Downie
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The latter.

Brake fluid adsorbs water from the air. The water will cause internal corrosion. In theory, it may also boil, turn to steam, and leave you temporarily with no brakes, although I've no direct experience of that happening.

Chris

Reply to
Chris Whelan
[...]

The average driver should be able to save 75 quid a year by more economical driving!

Chris

Reply to
Chris Whelan

I've heard and read about hydraulic fluid absorbing moisture from the air many times but the bit that always puzzles me is the hydraulics are a sealed system so how does the oil come into contact with the air/ moisture. Yes there is a breather in the top of of the reservoir but that has a rubber seal that only lifts slightly to account for pressure differentials. So again how does the oil in the pipes and pistons absorb this moisture?

Bob

Reply to
Bob Dodds

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

Reply to
Paul

It doesn't need very much and biderinal moving seals are rarely proof against both directions, every time something cools down there's a tiny negative pressure in the system.

Reply to
Duncan Wood

And the slack caused by the frictional bits wearing.

Reply to
Duncan Wood
[...]

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

Reply to
Chris Whelan

Sensible, and the cost is not high.

Reply to
Chris Bartram

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

Reply to
Bob Dodds

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

Reply to
Duncan Wood

e:

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

Reply to
Bob Dodds

it only needs a tiny amount of water to rot the bore of a steel master cylinder, I have seen dozens. completely unneeded and expensive, only caused by lack of regular fluid changes and exacerbated by people that keep brake fluid for umpteen years and use it to top up the master cylinder (never necessary if the car is correctly maintained.)

Reply to
Mrcheerful

OK I give in to people with more experience, it just seems strange to me that a virtually sealed system can be affected at its remote ends 2 to 3 meters down a micro bore system by the tiny breather hole at the reservoir which is sealed with a "rubber" relief valve.

Bob

Reply to
Bob Dodds

I'd agree if I hadn't seen loads of them.

Reply to
Duncan Wood

I think that's the only bit that corrodes, the reason silicone iisn't popular is that it isn't miscible & any water does end up in the piston/hot bit.

Reply to
Duncan Wood

OK I give in to people with more experience, it just seems strange to me that a virtually sealed system can be affected at its remote ends 2 to 3 meters down a micro bore system by the tiny breather hole at the reservoir which is sealed with a "rubber" relief valve.

Bob

the fluid at the 'bottom' end seems to draw in dampness past the seals in the cylinder or caliper, this is easily seen by the discolouration of the fluid, when you change it the fluid almost immediately changes colour, long before the fresh fluid could get there. at the top moisture pulls past the breather cap and seems to accumulate at the bottom of the master cylinder bore. happily the dampness rarely seems to effect abs units which are the furthest from moisture entry points.

Reply to
Mrcheerful

Most of our motorbikes have completely sealed systems in that there is a rubber concertina diaphragm at the master cylinder between the fluid and atmosphere that expands and contracts to follow the fluid volume changes (pad / shoe, wear / replacement etc).

However, I've also had to de-rust, de-corrode / replace quite a few calipers and drum pistons over the years.

T i m

Reply to
T i m

Indeed, but the volume changes will be much smaller on a motorcycle.

I'm guessing that the reason for them using a semi-sealed system is due to the likelihood of them spending some of their life in a non-vertical orientation. That's certainly the case with my Son's Gas-Gas trials bike...

Do you mean on cars, or motorcycles? I've never seen hydraulic drum brakes on a bike.

Chris

Reply to
Chris Whelan

True, but the volume of the reservoir is that much smaller as well.

Yup, that and difficult to be vented as they are often out in the weather.

Ah, yes sorry, a bit of blurring of machines in my head. The last bike I worked on was my old GPZ550 with a rear hydraulic disk and the master cylinder was the most corroded I thing I've ever seen in my life (other than a car battery terminal probably).

It all seemed a shame as well as the entire hydraulic system (reservoir, master cylinder and caliper) is about 50cm long. ;-(

Cheers, T i m

Reply to
T i m

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