Home | Mailing List | Specifications | Care and Feeding | Modifications | Vendors | Literature


Brake Fluid: A Hot Topic

By Larry Carley, Copyright 2000 CarleySoftware.com

Did you know the average motorist who drives 10,000 to 15,000 miles a year uses his brakes about 75,000 times a year?

Did you know that nearly half of all motorists in a recent Car Care Council survey said brake failure was their number one fear amongst driving emergencies?

Did you know that after three years of service, the average boiling point of the brake fluid has dropped to a potentially dangerous level because of moisture contamination and may not meet minimum federal requirements for brake fluid?

Did you know that probably half of all cars and light trucks that are 10 or more years old have never had their brake fluid changed?

Did you know that in many European countries, regular brake fluid checks are required, and that half of all cars routinely fail such tests?

Brake fluid is one of the most neglected fluid in vehicles today, yet is vitally important for safe driving. Consequently, there’s a real opportunity for brake specialists to address an issue that many motorists do not even realize exists. The issue is old brake fluid that may be unsafe due to moisture contamination. The solution is to test your customer’s brake fluid and recommend replacement when needed.


Many experts have long recommend changing the brake fluid every year or two for preventative maintenance. Their rationale is based on the fact that glycol-based brake fluid starts to absorb moisture from the moment it is put in the system. The fluid attracts moisture through microscopic pores in rubber hoses, past seals and exposure to the air. The problem is obviously worse in wet climates where humidity is high.

After only a year of service, the brake fluid in the average vehicle may contain as much as two percent water. After 18 months, the level of contamination can be as high as three percent. And after several years of service, it’s not unusual to find brake fluid that contains as much as seven to eight percent water.

Brake Fluid Chart

An NHTSA survey found that the brake fluid in 20% of 1,720 vehicles sampled contained 5% or more water!

As the concentration of moisture increases, it causes a sharp drop in the fluid’s boiling temperature. Brand new DOT 3 brake fluid must have a dry (no moisture) boiling point of at least 401 degrees F, and a wet (moisture-saturated) boiling point of no less than 284 degrees. Most new DOT 3 fluids exceed these requirements and have a dry boiling point that ranges from 460 degrees up to over 500 degrees.

Only one percent water in the fluid can lower the boiling point of a typical DOT 3 fluid to 369 degrees. Two percent water can push the boiling point down to around 320 degrees, and three percent will take it all the way down to 293 degrees—which is getting dangerously close to the minimum DOT and OEM requirements.

DOT 4 fluid, which has a higher minimum boiling temperature requirement (446 degrees F dry and 311 degrees wet) soaks up moisture at a slower rate but suffers an even sharper drop in boiling temperature as moisture accumulates. Three percent water will lower it’s boiling point as much as 50%!

Considering the fact that today’s front-wheel drive brake systems with semi-metallic linings run significantly hotter than their rear-wheel drive counterparts, high brake temperatures require fluid that can take the heat. But as we said earlier, the brake fluid in many of today’s vehicles can’t because it’s old and full of moisture.

Water contamination increases the danger of brake failure because vapor pockets can form if the fluid gets too hot. Vapor displaces fluid and is compressible, so when the brakes are applied the pedal may go all the way to the floor without applying the brakes!

In addition to the safety issue, water-laden brake fluid promotes corrosion and pitting in caliper pistons and bores, wheel cylinders, master cylinders, steel brake lines and ABS modulators.


From time to time we hear about reports of "unexplained" brake failures that caused accidents. When the vehicle’s brakes are inspected, no apparent mechanical fault can be found. The fluid level is normal, the linings are within specifications, the hydraulics appear to be working normally and the pedal feels firm. Yet the brakes failed. Why? Because something made the brakes hot, which in turn overheated the fluid causing it to boil. The underlying cause often turns out to be a dragging rear parking brake that does not release. But that’s another story.

The same kind of sudden brake failure due to fluid boil may occur in any driving situation that puts undue stress on the brakes: a sudden panic stop followed by another, mountain driving, towing a trailer, hard driving, etc.

A case in point: A child was killed in an accident when the five-year old minivan with 79,000 miles on it his parents were driving suffered loss of pedal and crashed while the family was driving in the mountains of Washington state. Fluid boil was blamed as the cause of the accident.


What do the auto makers say about fluid changes? General Motors and Chrysler do not mention brake fluid in their scheduled maintenance recommendations. A General Motors spokesman said Delco Supreme 11 DOT 3 brake fluid contains additives than many other brake fluids do not, so it is essentially a lifetime fluid. Starting in 1993, GM began using a new type of rubber brake hose with an EPM lining and outer jacketing that reduces moisture penetration by 50%. So GM does not consider fluid contamination to be a significant problem.

Ford, however, recently changed its position and now recommends fresh fluid every 36,000 miles or three years—and to replace the fluid each time the brake pads are changed.

Several import vehicle manufacturers also recommend fluid changes for preventative maintenance. BMW says the fluid should be changed every two years. Honda recommends a flush & fill every 25,000 to 30,000 miles. Subaru also recommends a 30,000 mile fluid change. Volkswagen recommends changing the fluid every two years, and clearly states this in their owners manuals.

If motorists would only follow this simple advice to change their brake fluid periodically, they could greatly reduce the risks associated with moisture-contaminated brake fluid. The could extend the life of their brake systems and likely save themselves a lot of money in the long run—especially if their vehicle is equipped with ABS (you know how expensive ABS modulators are to replace!).


Since you can’t tell how badly contaminated brake fluid is by its appearance alone (unless the fluid is full of rust or is muddy brown), the fluid should be tested unless you’re changing it for preventive maintenance or as part of a brake job.

There are three ways to check the level of moisture contamination in brake fluid:


When the fluid is changed, use the type of brake fluid (DOT 3 or 4) specified by the vehicle manufacturer.

As any brake fluid supplier will tell you, brake fluid is NOT a generic product. Just because a fluid meets the minimum DOT 3 or DOT 4 specifications doesn’t mean it can tolerate moisture or provide the same degree of corrosion protection as another brand of fluid.

Raybestos, for example, recently introduced a new "Super Stop Super High Performance" DOT 3 fluid with a dry boiling point of 550 degree F—which meets Ford’s latest requirements.

So the next time you’re inspecting or servicing a customer’s brakes, be sure to check the condition of the fluid as well as the level. If you add or change fluid, use type specified by the vehicle manufacturer (DOT 3 or 4) and use the highest quality fluid you can get. And above all, help educate your customers about the benefits of changing the brake fluid for preventative maintenance

Contact Information