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Plan Your Ride

Digital Brake Fluid Tester

Over time, brake fluids will take on water, break down, and compromise the safety of the brake system. The use of a digital brake fluid tester is ideal for testing the moisture content in any brake fluid sample. Developed for professional use in automotive servicing and repair workshops. Moisture content is indicated by the five LED's in terms of percentage of moisture in the brake fluid.

To check brake fluid quality
Unscrew the brake fluid reservoir; remove the cap from the brake fluid tester to reveal the testing forks.
Press the button on the end of the tester for power on, the green LED will light up to indicate the battery is OK.
Immerse the tester forks into the reservoir for approximately once second and the test results will be correctly indicated by the LED's.
Wipe of fluid residue before retest or power off.
The tester will automatically power off after 12 seconds in inactivity.

LED indication
GREEN: Battery OK, no water in the brake fluid
GREEN/YELLOW: Less than 1% of moisture content in brake fluid.
GREEN/YELLOW/YELLOW: Approximately 2% of moisture content in brake fluid.
GREEN/YELLOW/YELLOW/RED: At least 4% of water. Urgent change of liquid is necessary Danger!

Explaining the myths of Brake Fluid.
Most automotive professionals agree that glycol-based brake fluid, (DOT 3, DOT 4, DOT 5.1) should be flushed, or changed, every 1–2 years. Many manufacturers also require periodic fluid changes to ensure reliability and safety. Once installed, moisture diffuses into the fluid through brake hoses and rubber seals and, eventually, the fluid will have to be replaced when the water content becomes too high approximately 3 per cent.

DOT 5 is silicone fluid and the above does not apply. Ideally, silicone fluid should be used only to fill non-ABS systems that have not been previously filled with glycol based fluid. Any system that has used glycol based fluid will contain moisture; glycol fluid disperses the moisture throughout the system and contains corrosion inhibitors. Silicone fluid does not allow moisture to enter the system, but does not disperse any that is already there, either. A system filled from dry with silicone fluid does not require the fluid to be changed at intervals, only when the system has been disturbed for a component repair or renewal.
DOT 5 is a silicone based synthetic fluid originally designed for use in military and government vehicles where regular maintenance could be problematic. Although not hygroscopic in the traditional sense, water molecules will still find their way into your hydraulics creating globules rather than being suspended as with traditional DOT 3/4 & 5.1 fluids…the end result is the same: contamination. The United States armed forces have standardised on silicone brake fluid since the 1990s. Silicone fluid is used extensively in cold climate, particularly in Russia and Finland.

DOT 5 Silicone was never intended for serious high performance application. However Harley Davidson uses DOT 5 on many of their early model bikes, I suspect because it won't ruin your paint if you're all thumbs and miss the toilet a lot. Interesting aside; HD's sporting wing; Buell, has switched to a higher performance DOT 4 in all their 2000 models. DOT 5.1, albeit confusing in designation (thank the D.O.T. for that), is effectively a lighter viscosity synthetic poly glycol fluid with DOT 4 performance parameters. It was developed for use in mechanical ABS systems that require a higher cyclic rate capability.

A small drop in brake fluid level in the master cylinder reservoir can be "topped up" but if the level consistently drops, the cause should be investigated and repaired. Brake fluid level in the master cylinder will drop as the linings (pads or shoes) wear and the calipers or wheel cylinders extend further to compensate. Overspill from pushing back pistons should be avoided, because glycol based fluid will quickly lift or strip paints and other coatings on contact (it can be removed by quickly washing with water, not wiping). Brake fluid level may also be low because of a leak, which could result in a loss of hydraulic pressure and consequently a significant loss of braking ability.

Brake fluids with different DOT ratings cannot always be mixed. It must be of the same type, and at least the same rating. DOT 5.1 can replace DOT 4 and 3, DOT 4 can replace DOT 3.
DOT 5 should not be mixed with any of these as mixing of glycol with silicone fluid may cause corrosion due to trapped moisture.

Brake fluid is toxic and combustible and can damage painted surfaces.

Boiling point
Brake fluid is subjected to very high temperatures, especially in disk brake calipers. It must have a high boiling point to avoid vaporizing in the lines. This vaporization is a problem because vapor is compressible and negates hydraulic fluid transfer of braking force.

Quality standards refer to a brake fluid's "dry" and "wet" boiling points. Wet boiling point, which is usually much lower (although above most normal service temperatures), refers to the fluid's boiling point after absorbing a certain amount of moisture.

This is several (single digit) percent, varying from formulation to formulation. Glycol-ether (DOT 3, 4, and 5.1) brake fluids are hygroscopic (water absorbing), which means they absorb moisture from the atmosphere under normal humidity levels. Non-hygroscopic fluids (e.g. silicone/DOT 5-based formulations), are hydrophobic, and can maintain an acceptable boiling point over the fluid's service life, although at the cost of potential phase separation/water pooling and freezing/boiling in the system over time - the main reason single phase hygroscopic fluids are used.

Boiling points for common braking fluids,

Dry boiling point

Wet boiling point


190 °C (374 °F)

140 °C (284 °F)


205 °C (401 °F)

140 °C (284 °F)


230 °C (446 °F)

155 °C (311 °F)


260 °C (500 °F)

180 °C (356 °F)

DOT 5.1

270 °C (518 °F)

190 °C (374 °F)

March 30, 2013

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