Refractometer for
Testing Brake Fluid

Brake Fluid’s Dirty Little Secret:

DOT Standard 116 requires that DOT3 brake fluid, with no moisture content, will only boil at temperatures above 400 °F (205 °C). The brake fluid in a three to four year old car, with three to four percent water content in the brake fluid, could boil at less than 300 °F (149 °C).

Easy To Use:       

Testing brake fluid on a refractometer is simple and straightforward. Just add a drop of brake fluid from the master cylinder or wheel cylinder on the measuring surface, close the sample cover, allow a brief period for the temperature to stabilize, and press the <GO> button. The refractometer will quickly display your measurement of water content or estimated boiling point.

Refractometer for Testing Brake Fluid

Measure brake fluid water content and estimated boiling point for DOT3 and DOT4 brake fluids. 

As part of the regular maintenance interval, many automobile manufacturers recommend routine brake fluid inspection. However, there was never an easy, inexpensive means for testing it; until now. A Brake Fluid Refractometer represents the most precise field testing means available to determine the water content and estimated boiling point of automotive brake fluids.

No Brakes – The Dangers of High Water Content in Brake Fluid:

A dangerous condition exists when brake fluid boils in the braking system; it can feel like air trapped in the braking lines. In extreme cases, the driver may push the brake pedal entirely to the floor without slowing or stopping the vehicle. In colder climates, brake fluid with high water content becomes thick, causing slow pedal response or difficulty braking. Moreover, as the water content in brake fluid increases, it will become more corrosive to steel pistons and the ABS modulator.

Even New Cars Contain Water in Their Brake Fluid!

Automotive brake fluid is “Hygroscopic.” That means that it won’t take long for brake fluid to absorb moisture directly from the air, even in a sealed braking system. The Society of Automotive Engineers (SAE) conducted a study of automotive brake fluid. They found that an average one-year-old car had 2% moisture in its brake fluid. Randomly tested cars in the U.S., with an average age of eight years, showed an average water content greater than 2½%. Of these cars, 25% had a dangerously high water content of 4% or more.

The Result is Catastrophic Brake Failure!

Brake fluid with high water content has a reduced boiling point. That means that the brake fluid will boil in the wheel cylinder at a lower temperature than specified. When traveling down a steep grade, towing a trailer, or in the event, a brake pad becomes stuck against a rotor, brake fluid temperature can increase high enough to boil if it contains enough water. In the case of boiling, stepping on the brake pedal will now only compress the vapor instead of applying hydraulic force to the pad – the result is sudden brake failure.

When Should I Change my Brake Fluid?

Generally, the type of driving should influence when to change brake fluid. If the vehicle is used for towing, driving in mountainous regions, driven at high speeds or has ABS brakes, it would be best to change the fluid at 2% water content. All vehicles should have the brake fluid changed when the water exceeds 3%.

Many motor vehicle companies recommend changing brake fluid every 12 months, others recommend changing every 24 months. The amount of moisture that brake fluid absorbs depends on a number of factors including the type of fluid, humidity, type of brake hoses, condition of the seals and mileage.

Generally, brake fluid will absorb 1% or more moisture per year of service life. A two year old car will have 2-3% water in the brake fluid. 2% water reduces the boiling point of DOT3 brake fluid by approximately 135 degrees Fahrenheit (75 degrees Celsius). 2% water in DOT4 brake fluid reduces boiling point by 81 degrees Fahrenheit (45 degrees Celsius).

Generally, brake fluid will absorb 1% or more moisture per year of service life. A two year old car will have 2-3% water in the brake fluid. 2% water reduces the boiling point of DOT3 brake fluid by approximately 135 degrees Fahrenheit (75 degrees Celsius). 2% water in DOT4 brake fluid reduces boiling point by 81 degrees Fahrenheit (45 degrees Celsius).