Brake fluids are not only expected to work in normal and in extreme conditions but these are also called on to do other functions.

  • Brake fluid also lubricates the moving parts in the brake system
  • Brake fluid must be compatible and not damage rubber seals and hoses
  • Also equally important to boiling point, brake fluid protects the metal (steel and copper) components of the brake system from corrosion.

The metal components of a brake system are subject to wear and corrosion. This is a fact of life and it goes without saying that the ability of the brake system to function properly is impaired if the master cylinder, slave cylinders and valves, which are made of steel, are worn and corroded. The effects of corrosion are not immediate and the deleterious effects are cumulative in nature. Cumulative effects over the long term can affect braking efficiency and responsiveness, putting drivers at risk especially during emergency braking situations. Advance corrosion will eventually lead to diminished braking performance then eventually brake failure. Copper, of which the tubes and ABS internal parts are made of, are also subject to corrosion. Copper plating (when copper particles are suspended in the fluid and starts “plating” into steel parts) can interfere with the proper operation of the ABS system and will eventually result in the failure of the ABS. ABS systems as we know are very expensive to replace.

Brake fluids contain corrosion inhibitors and the most common ones used are called amines. These amines work in two basic ways: 1) by neutralizing acids in the fluid and this is measured in terms of reserve alkalinity and 2) by forming a protective barrier to repel water on the metal surfaces thereby preventing the metal from interacting with water. The second function is very important as amines are not always able to neutralize all acids in the fluid. A protective barrier is always required to protect the metal parts from any underlying acid in the system. Unfortunately, pH meters (which can be used to measure levels of corrosion protection) are only able to measure reserve alkalinity so there is no way of knowing whether there is enough amine in the system to form that very important protective barrier.

Brake Fluid Deterioration

In summary, there are two ways with which your brake fluid can deteriorate with age and lose its effectiveness:

The first would be the ingestion of too much moisture which leads to a severe drop in the brake fluid’s boiling point. Many OEMs recommend brake fluid changes as a form of preventive maintenance and most of their recommendations range from 24 to 30 months.

The second would be the depletion of the brake fluid’s corrosion inhibitor or its ability to protect the metal parts from corrosion. The brake fluid’s continued ability to provide corrosion protection is gaining traction as a way of determining the brake fluid’s suitability for further use. Like most chemical reactions, the depletion of a brake fluid’s corrosion inhibitors are accelerated with elevated temperatures.A car driven hard with frequent use of brakes, will very likely have a faster depletion of its brake fluid’s corrosion inhibitors.

Testing of Brake Fluids in Service

To reduce risks associated with brake failures, it is recommended that the brake fluid in your car is tested regularly for moisture contamination and/or depletion of corrosion inhibitors. Fresh brake fluid comes in various colours but all these are generally clear liquid when new. These turn dark in service but a dark colour is not an indication that the brake fluid is due for replacement. There are several testing methods out there and we will look closely at each one.

1. Measurement of Moisture Levels

a. Brake Fluid Refractometer – this is an optical instrument that measures the fluid’s moisture content by the way the fluid “bends” light. This is an accurate instrument but its scales need calibration for DOT 3 or DOT 4 fluids.

b. Brake Fluid Tester – commonly used in a lot of workshops as it is relatively cheap and easy to use. It measures the fluid’s conductivity to give inferred readings on its moisture content. However, brake fluids come in various formulations and consequently have differing levels of conductivity. The use of this type of tester is therefore prone to errors. It comes as no surprise that these types of testers give “fail” readings even for brand new brake fluids. These also need to be calibrated for each brand and grade of brake fluid.

c. Water Test Strip – these are test strips that indicate the presence of moisture in the brake fluid. These failed because they were too sensitive. Even moisture in the air activated the strips even before these were dipped into the fluid.

d. Boiling Point Tester – this is regarded as the most accurate method as the instrument actually boils a small sample of the brake fluid and measures the boiling temperature. Its accuracy is within a few degrees to the results of laboratory equipment and is quite easy to use. The instrument is connected to the car’s battery and dipped into the reservoir or in a small container containing a sample of the fluid. There is currently no standard guideline on the minimum boiling point but it would be prudent and a good practice to change the brake fluid when the boiling point gets below 180°C. Any boiling point below that increases the likelihood of vapour lock during hard braking. This test is very important as it immediately tells you that your margin of safety has been greatly diminished by your brake fluid’s highly reduced boiling point.

2. Measurement of Corrosion Inhibitors

a. Reserve Alkalinity – there is no accurate portable way of measuring reserve alkalinity and this test would most likely be done in a laboratory. Furthermore, it is only able to measure the fluid’s ability to neutralize acids but will not be able to indicate if the fluid still has the ability to provide a protection barrier to the metal parts of the brake system.

b. Copper Test Strips – this method is gaining popularity in the United States. It is very easy to use and gives a good picture of the state of your in-service brake fluid. Note that fresh brake fluid has nil copper content in it. Continued use of brake fluid that no longer provides adequate corrosion protection has long term implications on the performance of your brake system. In this test, a test strip is dipped into the brake fluid and a change in colour to the strip is compared against a colour chart. The colour chart indicates whether the fluid is still providing corrosion protection to the brake system. The theory of this test is that when copper molecules start being suspended in the brake fluid, it is an indication that the fluid has lost its ability to protect the system from corrosion. Steel usually corrodes ahead of copper so when copper starts showing up in the fluid, then it is a sign of imminent danger. The presence of copper in brake fluid usually precedes the onset of the corrosion of the steel components – which in a brake system comprise the critical moving parts and have close tolerances.


From the workshop point of view, the tester has to be an effective selling tool – a tool that gives credibility when making recommendations. We want a tester that actually helps convince people that their safety is at risk with continued use of their in service brake fluid. In addition to this we also want a tester that is easy to use, accurate and reliable and cost effective

The boiling point tester is recommended when we want to determine the level of moisture contamination. It is the most accurate of all portable testing equipment. The initial investment could be in a few hundred dollars but each use would cost virtually nothing. A few brake fluid replacement jobs would allow recovery of the initial investment.

Copper test strips can be used to ensure that brake fluid in the system still provides corrosion protection. These come in bottles of a hundred strips. Initial investment is substantially lower than that of a boiling point tester and the investment can be recovered in 2-3 fluid replacement jobs. These need to be replenished when they run out.

Again, it cannot be overemphasized that the regular testing of brake fluid in service is not only a good way to generate extra business for workshops but it may well be your contribution to keeping our roads safer, potentially saving lives!

For further information:

For more information free call the Bendix Brake Advice Centre on 1800 819 666 or +61 3 5327 0211 from overseas (8am-5pm Monday to Friday EST), e-mail us at:

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