The braking system isn't sealed air tight. The fluid changes color over time because it picks up moisture from the surrounding air. That's why it needs to be flushed regularly, that moisture lowers the boiling point of the fluid. When the pads wear the fluid level in the master cylinder drops, air is allowed in to prevent a vacuum from forming in the system.

The cooling system isn't air tight either. The cap is essentially a one way valve. When the coolant heats up the cap seals allowing pressure to build, as it cools down the cap allows air in to prevent a vacuum from forming within the system.

 

The cap has a rubber diaphragm. When the fluid level drops or there is a vacuum in the system the diaphragm sucks in. So the cap does let air in but it does not get past the diaphragm.

Now technically whenever you do a brake pad replacement you are SUPPOSED to crack the bleeder screw on each caliper before collapsing the pistons. This way any rubber that has settled to the bottom of the calipers is not pushed back up into the ABS HCU which can potentially damage it. If you follow this practice every time you replace the pads you should never have to flush the brake system.

 

you are correct... for older vehicles...

newer vehicles have a small slit in the diaphragm that lets air in as the fluid is pushed to the brakes. But the slit does not let air flow without vacuum (or pressure) in the reservoir, so it should keep most moisture out of the system.

and water does NOT make brake fluid dark. The fluid will get darker for 2 main reasons - 1 is due to the heat of the engine causing the additives to darken, and 2 is that you will get microscopic pieces of rubber wearing from the seals.

 

Brake systems are sealed...but not perfectly sealed. They can draw moisture from the rubber lines and other porous components.

Many vehicle makers use DOT 3 type fluid although there may be some who prefer to use DOT 4 type fluid because this fluid has a boiling point that is higher. These two fluids are both glycol-based and are hygroscopic, this term simply put means they will both absorb water. If a container is left open, it absorbs moisture from the air, whilst the brake system, over years, will slowly absorb water through rubber hoses and other parts.

There is another brake fluid that is silicone based. This is classed as DOT 5.0 or 5.1 types. These fluids have a blue dye put in that makes it visibly different to other brake fluids. The advantage of silicone-based brake fluid is that it isn't hygroscopic and therefore won't cause damage to any painted parts; however, it will pass through smaller pores and won't provide as good a seal. The majority of vehicle manufacturers prefer to use DOT 3 fluid in their factories. Mixing two different types of fluids should never be done. Those who wish to use a silicone based brake fluid will have to clean the entire system of glycol-based fluid.

As mentioned before, water in brake fluid can cause corrosion, leaving deposits of sludge in the brake system. Many also believe that dark colored brake fluid should be changed to a light amber color. AMRA may surprise a lot of people with their surprise findings and for many sorts out the difference of opinion.

The myth regarding the color of brake fluid can quickly be solved in that this is not an indicator of the brake fluids quality. The color of several brake fluids can change naturally when the fluid is in contact with the brake systems rubber components. Tests that have been carried out shown that dark is still good and light amber fluid was poor.

Brake fluid is hygroscopic and having water in the brake fluid will lower the boiling point and therefore may not work as well. Water causes problems with corrosion but there are no tests available to check the water content of brake fluids. Through research, it has been found only a small amount of water is absorbed into the brake system. The rubber materials have improved greatly, therefore helping the seal to keep moisture out a lot better. The plastic see- through reservoirs limits the brake system being open to the air.

It is corrosion inhibitors in brake fluid that help to reduce breaking down of materials in the brake system. These inhibitors eventually wear out with age, water content, heat from the high braking system, and the copper in the brake lining will start to corrode. As the amount of copper ions build up in brake fluid, these act as oxidizers and other parts will begin to corrode. These can be the ABS valves and the master cylinders.

It is possible to check for copper content using paper strips that will change color if there is a high content of copper. Most service shops may use these but there may be some who don't. If not, then they won't actually know whether the brake fluid is good or should be changed. To be on the side caution in a situation like this, it may be better to have the brake system flushed and clean brake fluid put in. This can save you a lot of money as corrosion related repairs are very expensive and more important, you want to be sure your brake system stops works as you want it to.



source: http://ezinearticles.com/?The-Import...cks&id=1712629

 

I have used a power bleeder by motive products for years. It attatches to the reservoir and puts pressure on it while you bleed all of the calipers. It's worth the money and works with most vehicles.

 

good info on brake fluid here.

 

I'm headed toward all new brakes on my 2004 Screw 4x4. I will agree that brake fluid should be changed every few years or so and have been delinquent with my now 6 year old truck. Early on in this thread someone said the dealer should do it because of the ABS? Is this true? I was going to let my regular tire and brake shop handle it.

 

The dealer doesn't have to do it, any competent shop can handle it. The process is the same as it would be for bleeding the brakes. The only problem that can happen is if air should get sucked into the ABS module, if the bleeder is left open and you let go of the pedal. Then a scan tool needs to be used to cycle the module in order to push the air out.