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Bendix Brake Fluid 


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 Post subject: Bendix Brake Fluid
Posted: Sat Jun 01, 2013 1:30 am 
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The Importance of Brake Fluid

In one year, most of us drive our cars a minimum of 15,000 to 20,000 kilometres. During this time many components of our cars need regular maintenance, our engine oil changed, coolant levels checked, wiper blades replaced and even our brake pads and rotors checked. Yet one thing that is often overlooked is our brake fluid. Brake fluid is an important part of our car and it must be regularly checked and maintained. Many OEM and maintenance experts recommend that we change our brake fluid at least every two years. We’ll tell you why.

What is brake fluid?

Brake fluid is basically a hydraulic fluid that is responsible for transferring the force you put on the brake pedal to the brake pads and rotors. Without it, the braking system in our car cannot function. It transfers force so naturally it must have important properties for it to perform completely. These properties include:

  • Being non-compressible
  • Have a high boiling point
  • Remain a liquid even when freezing

However, brake fluid by nature absorbs moisture from the atmosphere through microscopic pores in the brake lines and through the small vent in the reservoir. In fact brake fluid beings to take in moisture the moment you open the bottle and pour it into your braking system.

Surprisingly only after 12 months, the brake fluid will have absorbed 2% of water; it increases to 3% only after 18 months. It might not seem like much but 2% of water will decrease the boiling point by 75°C. As more moisture is absorbed, the boiling point dramatically deceases thus increasing the risk of brake failure. Moisture also alters the viscosity of the brake fluid, this is important as it affects the performance of the Anti-lock brake systems (ABS).

A spongy brake pedal is a sign that the brake fluid has absorbed moisture.
This is the reason why there have been many cases of brake failures even if the braking systems were found to be mechanically in order.


DOT 4 and DOT 3 Brake Fluids

America’s Department of Transportation (DOT) has put a standard in place for brake fluids with two standards in the industry – DOT 4 and DOT 3. Although there are other types of brake fluids, DOT 4 and DOT 3 are used with most vehicles so we’ll stick with these two for purposes of this discussion. Note the Australian standard has a stricter boiling point requirement the internationally accepted DOT standard. Regardless, the Bendix line of brake fluids not only conforms to both the DOT and Australian standards but also exceeds these standards by a wide margin.

The main difference between DOT 4 and DOT 3 is that DOT 4 has a higher boiling point therefore is usually recommended by car manufacturers. DOT 4 also has slightly better viscosity characteristics and it absorbs moisture slower than DOT3. The only disadvantage is when it does absorb moisture; the drop in boiling temperature is higher than DOT 3. So DOT 4 brake fluids offer more stability and safety for a longer period of time but would need to be replaced at the recommended OEM intervals.

Some cars had their brake systems designed before the introduction of the DOT 4 fluid so the material of their brake hoses are not compatible with some DOT 4 brake fluid during testing. Although in real life conditions, it is quite difficult to observe the same effects. Because of this some Australian car manufacturers still recommend the use of DOT 3 brake fluid but most makers still recommend the use of DOT 4 because the extra safety it provides outweigh the minimal chance of the hose failing.

The role of brake fluid isn’t only to work properly in extreme conditions but it also acts as lubrication for the moving parts and stops corrosion from occurring in the braking system. The brake fluid contains corrosion inhibitors that form a protective layer that keeps water off metal surfaces achieves this.

Brake Fluid Deterioration

As with anything, brake fluid also deteriorates with age. This is through the increase in moisture being absorbed into the fluid and also the loss of the corrosion inhibitors. These two ways lead to the loss of performance in brake fluid. The performance loss is also associated with frequent use, so if a car is driven hard frequently, the negative effects are increased.

Therefore it is important to test your brake fluid regularly for the negative effects. Although brake fluid is usually a clear liquid that comes in various colours, a dark colour does not necessarily mean it is time for replacement. There are a few testing methods that are more accurate for determining if the brake fluid is due for replacement or not.

Measurement of Moisture Levels

Brake Fluid Refractometer – It is accurate but it needs to be calibrated for DOT 3 or DOT 4 fluids.

Brake Fluid Tester – commonly used in workshops since it’s cheap and easy to use but it is not as accurate due to its errors. This tester also needs to be calibrated.

Water Test Strip – not as effective because it was too sensitive, even in air the strips indicated moisture was present.

Boiling Point Tester – regarded as the most accurate method to determine moisture levels. It boils a small sample of the brake fluid and measures the boiling temperature. Anything below 180°C indicates that it’s due for replacement because safety is greatly reduced for any boiling point below that.

Measurement of Corrosion Inhibitors

Reserve Alkalinity – there’s no portable way to measure this accurately so it is usually done in a laboratory. It also isn’t accurate as it only gives an indication of the fluid’s ability to destroy acids but not its ability to provide a protection barrier to the metal parts in the brake system.

Copper Test Strips – it’s quite easy to use and gives a fairly accurate indication of the copper content in your brake fluid. Fresh fluid has zero copper content so an increase in copper content would indicate that the fluid is losing it’s ability to combat corrosion.


The most effective test for moisture is the boiling point tester as it is the most accurate of all portable methods. The initial cost might be high be could be used multiple times thereafter.

To measure the corrosion inhibitors of brake fluid the copper test strips are recommended as it gives a definite indication of how well your brake fluid is preventing corrosion in the system. The initial cost is less than the boiling point tester but they need to be replaced when they run out.

Again, it cannot be stressed enough that regular testing and maintenance of brake fluid is critical for keeping our roads safer, potentially saving lives!



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