[E90330iS]

If not, how often do you do yours?

If so, what does it really prevent?

[fdriller9]

As your brake fluid ages it retains water which compresses at a different rate than the brake fluid.

Water compress more easily so you get a softer pedal feel and if there is too much water/air in the lines...your brakes will not work.

So yes it is necessary. And I swap my fluid as per the BMW CBS plan, which is 2 years as you stated.

EDIT: I don't use BMW fluid though...ATE blue/gold.

[F32Fleet]

Originally Posted by *********

As your brake fluid ages it retains water which compresses at a different rate than the brake fluid.

Water compress more easily so you get a softer pedal feel and if there is too much water/air in the lines...your brakes will not work.

So yes it is necessary. And I swap my fluid as per the BMW CBS plan, which is 2 years as you stated.

+1

And the water will also start to cause rust.

[raceyBMW]

+1 to all that was said.

In addition, water also has a much lower boiling point than brake fluid....and boiling water in the break lines = air

[Priority 1]

Originally Posted by raceyBMW

+1 to all that was said.

In addition, water also has a much lower boiling point than brake fluid....and boiling water in the break lines = air

I almost hate to do this because its not constructive at all, but what does boiling water equal?

[Tee212]

Its just common practice to flush the fluid, the moisture tends to build up inside the system and with the moisture theres oxygen which relates to the spongy brake pedal feel from compressing air.

[fdriller9]

Originally Posted by Priority 1

I almost hate to do this because its not constructive at all, but what does boiling water equal?

Evaporation....which also releases air.

You know what he meant....

[ENINTY]

Originally Posted by *********

As your brake fluid ages it retains water which compresses at a different rate than the brake fluid.

Water compress more easily so you get a softer pedal feel and if there is too much water/air in the lines...your brakes will not work.

So yes it is necessary. And I swap my fluid as per the BMW CBS plan, which is 2 years as you stated.

EDIT: I don't use BMW fluid though...ATE blue/gold.

So let me make a few points on this quote since it is full of mis-information...

First, water does not compress. Brake fluids do not compress. They do not compress at different rates. The non-compression of fluids is the operating principle behind hydraulic brake systems.

Second, brake fluid over time absorbes a small amount of water (from the water vapor present in the atmosphere). Brake fluid needs to be replaced periodically to remove the water in the brake system.

Third, water in the brake fluid lowers the boiling point of the brake fluid. When the brake fluid gets hot enough to boil, gas forms in the brake fluid (not air from outside the system), which causes the brake pedal to become soft (because gases do compress) and make the braking system less effective in applying pressure to the brake pads, which means less friction between the pads and the rotor.

Water in the brake system can form rust and cause sticky caliper and master cylinder pistons.

[Tee212]

Originally Posted by ENINTY

So let me make a few points on this quote since it is full of mis-information...

First, water does not compress. Brake fluids do not compress. They do not compress at different rates. The non-compression of fluids is the operating principle behind hydraulic brake systems.

Second, brake fluid over time absorbes a small amount of water (from the water vapor present in the atmosphere). Brake fluid needs to be replaced periodically to remove the water in the brake system.

Third, water in the brake fluid lowers the boiling point of the brake fluid. When the brake fluid gets hot enough to boil, gas forms in the brake fluid (not air from outside the system), which causes the brake pedal to become soft (because gases do compress) and make the braking system less effective in applying pressure to the brake pads, which means less friction between the pads and the rotor.

Water in the brake system can form rust and cause sticky caliper and master cylinder pistons.

lol dam you got him pretty good there lol @ water compressing

[Bcube]

OP,
And to continue the brake fluid replacement thought - two years is the recommended time with such fluids as BMW branded fluid, ATE 200, Castrol LMA, and some DOT 5.1 spec. These fluids are designed for low moisture absorption to meet two year replacement intervals under street conditions. They work very well for their intended usage. However, if one decides to get into the specialty "racing" fluids, be aware that these types of fluids have very hydrophilic characteristics and absorb water like a sponge. These types of fluids require frequent changes to keep your brake system healthy.

[ENINTY]

And just to add... Keeping fresh fluid in the system will keep the ABS modulator functioning properly. The E9x brake system is very complex because the software provides a set of functions that rely on the ABS module working properly like brake force distribution, stability control, hill hold, etc. None of these functions operate by the driver pushing the brake pedal, but by the pump in the ABS module and control of the valve bodies inside the ABS module. Getting rust particles lodged in there would not be good.

[bmw6953]

Yes, change it every two years! on any car for that matter.

[txusa03]

Originally Posted by bmw6953

Yes, change it every two years! on any car for that matter.

this is the best short answer!

[Chriztofor]

Agree with everyone about water absorption. Is it really that bad changing brake fluid every two years? For that matter changing coolant, power steering fluid, transmission fluid wouldn't be a bad idea either. Cars like clean fluid.

[Outlaw 06]

Originally Posted by ENINTY

So let me make a few points on this quote since it is full of mis-information...

First, water does not compress. Brake fluids do not compress. They do not compress at different rates. The non-compression of fluids is the operating principle behind hydraulic brake systems.

Second, brake fluid over time absorbes a small amount of water (from the water vapor present in the atmosphere). Brake fluid needs to be replaced periodically to remove the water in the brake system.

Third, water in the brake fluid lowers the boiling point of the brake fluid. When the brake fluid gets hot enough to boil, gas forms in the brake fluid (not air from outside the system), which causes the brake pedal to become soft (because gases do compress) and make the braking system less effective in applying pressure to the brake pads, which means less friction between the pads and the rotor.

Water in the brake system can form rust and cause sticky caliper and master cylinder pistons.

+1. All liquids are incompressible per the laws of fluid dynamics. Water has a much lower boiling point than brake fluid and it also causes corrosion so that's why it's prudent to periodically flush your brake lines.

[Dr. G]

I'm about 4 months overdue for a flush.

Is this a matter of concern, or would pushing back another month be acceptable?

[cb1111]

Originally Posted by Dr. G

I'm about 4 months overdue for a flush.

Is this a matter of concern, or would pushing back another month be acceptable?

Of course. Just get it done as soon as you can.

[fdriller9]

Originally Posted by ENINTY

So let me make a few points on this quote since it is full of mis-information...

First, water does not compress. Brake fluids do not compress. They do not compress at different rates. The non-compression of fluids is the operating principle behind hydraulic brake systems.

Second, brake fluid over time absorbes a small amount of water (from the water vapor present in the atmosphere). Brake fluid needs to be replaced periodically to remove the water in the brake system.

Third, water in the brake fluid lowers the boiling point of the brake fluid. When the brake fluid gets hot enough to boil, gas forms in the brake fluid (not air from outside the system), which causes the brake pedal to become soft (because gases do compress) and make the braking system less effective in applying pressure to the brake pads, which means less friction between the pads and the rotor.

Water in the brake system can form rust and cause sticky caliper and master cylinder pistons.

Fail on my part ....

Not to be a dick or anything but water IS compressible.....it just takes so much effort to compress it just a little bit that many consider it incompressible.

[ENINTY]

Originally Posted by *********

Fail on my part ....

Not to be a dick or anything but water IS compressible.....it just takes so much effort to compress it just a little bit that many consider it incompressible.

I'll stick with Pascal's law in this instance. Your initial post implied that the difference in compressibility between water and brake fluid was the cause of a soft brake pedal, which is totally untrue. In a hydraulic system fluids do not compress; the pressure is constant and it transfers force.

The effects of water in the brake fluid are it causes rust and reduces the effective boiling point of brake fluid. When the brake fluid boils it is less effective in transferring force, which increases stopping distances.

[subieworx]

While technically speaking this is true in racing the compressibility of different fluids is often considered.

Example, brembo lc600 and mogul rbf600 have very similar boiling points, but the brembo fluid results in a stiffer pedal feel especially as the fluid starts to heat up.

[fdriller9]

Originally Posted by ENINTY

I'll stick with Pascal's law in this instance. Your initial post implied that the difference in compressibility between water and brake fluid was the cause of a soft brake pedal, which is totally untrue. In a hydraulic system fluids do not compress; the pressure is constant and it transfers force.

The effects of water in the brake fluid are it causes rust and reduces the effective boiling point of brake fluid. When the brake fluid boils it is less effective in transferring force, which increases stopping distances.

Lol I admitted I was wrong....

[GB-335xiCoupe]

Just to clarify, and I know for a fact this to be true - liquids DO compress. Thus the term bulk compression modulus of liquids. All matter is compressible - even solids.

Bulk Modulus

http://www.theleeco.com/engineering/...lk-modulus.cfm

Bulk Modulus is a measure of the resistance of a fluid to compression. It is defined as the ratio of pressure stress to volumetric strain. The value of bulk modulus equals the pressure change x 100 required to cause a one percent change in volume.

However, obviously, as compared to gases, liquids are not very compressible. And in a system like a car's hydraulic braking system, the pressures aren't high enough to compress the fluids appreciably. That said, the boiling of entrained water is really the problem in brake fluid, as others stated above. The entrained water lowers the boiling point of the "mixture" of brake fluid and water. If that boiling point is reached, water vapor (not air) gets entrained in the the brake lines, leading to mushy brakes, the piston can't apply enough force to your calipers, and people die.