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Why Change Brake Fluid?

by Larry Carley copyright

Brake Fluid is a hot topic because most people don't know why it should be changed. 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?

So consider this: 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.

Probably half of all cars and light trucks that are 10 or more years old in the U.S. have never had their brake fluid changed. Yet in many European countries, regular brake fluid checks are required, and half of all cars routinely fail such tests. That's a good case for changing brake fluid.

Brake fluid is stored in the master brake cylinder reservoir. The reservoir is usually a translucent plastic container on top of the master cylinder. This allows you to see the fluid level inside without having to remove the filler cap. Opening the cap unnecessarily should be avoided because it allows moist air to enter the reservoir, and moisture is not good for brake fluid.

master brake cylinder fluid reservoir


Brake fluid is one of the most neglected fluid in vehicles today, yet is vitally important for safe driving. Consequently, you should check the fluid level regularly. The level will gradually drop as the brake pads wear, but a sudden drop in the fluid level usually means there is a leak in your brake system. The fluid level should be maintained between the ADD and FULL marks, or the MIN and MAX marks. If the level is low, add the type of fluid specified on the filler cap (DOT 3 or 4).


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 is 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 the 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 cannot because it is 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, for a time, recommended fresh fluid every 36,000 miles or three years, and to replace the fluid each time the brake pads are changed. Currently, however, Ford specifies no specific time or mileage recommendation for changing the brake fluid.

As for Chrysler, the only recommendation they publish is to change the fluid every 24 months on their Sprinter van.

A number of import car makers do recommend brake fluid changes for preventive maintenance at specific time/mileage intervals:
Acura: 36 months
Audi: 24 months
BMW: 24 months, or when indicated by Service Inspection Indicator
Honda: 36 months
Jaguar: 24 months all models except 2009 XF (36 months)
Land Rover: 36 months
Lexus: 36 months or 30,000 miles, which ever comes first
Mercedes-Benz: 24 months
MINI 24 months
Saab: 48 months (all models except 9-7X)
Smart: 24 months or 20,000 miles, which ever comes first
Subaru: 30 months or 30,000 miles (normal service) or 15 months/15,000 miles (severe service)
Suzuki: 24 months or 30,000 miles, which ever comes first (Forenza & Reno), 60 months or 60,000 miles (Grand Vitara and SX4)
Volkswagen: 24 months (New Beetle, City Gold, City Jetta), 36 months (all other models except Routan)
Volvo: 24 months or 37,000 miles (Normal), or 12 months (severe service)
Source for fluid change recommendations: Vehicle Manufacturer service information & 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. You can extend the life of your brake system and likely save yourself a lot of money in the long run on brake repairs, especially if your vehicle is equipped with ABS (because ABS modulators are very expensive 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 are changing it for preventive maintenance or as part of a brake job.

There are three ways to check the condition of the brake fluid:


brake fluid reservoir cap When the fluid is changed, use the type of brake fluid (DOT 3 or 4) specified by the vehicle manufacturer. The cap on the fluid reservoir will usually indicate what type of brake fluid is required. You can also find this information in your Owners Manual (look under brake fluid).

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 does not mean it can tolerate moisture or provide the same degree of corrosion protection as another brand of fluid. The wet and dry boiling points of any given brand of brake fluid will depend on the additives in the fluid. Dry boiling temperatures for commonly available DOT 3 brake fluids may range from just over 400 degrees up to 573 degrees F, which is quite a spread. Wet boiling temperatures for DOT 3 fluids don't vary as much, ranging from 284 to 313 degrees F. All of these brake fluids meet the minimum DOT requirements as well as OEM requirements, but obviously a fluid that has a higher dry and wet boiling temperature provides a greater margin of safety and heat resistance.

As for differences in DOT 4 brake fluids, advertised dry boiling temperatures can range from a low of 446 to a high of 646 degrees F depending on the brand. Wet boiling temperatures for DOT 4 fluids are typically in the 300 to 400 degree range, with some being over 430 degrees F. Again, there is quite a range in performance depending on the brand.

There are also high temperature glycol based DOT 5.1 brake fluids (not to be confused with DOT 5 which is silicone based). The dry boiling temperature rating for DOT 5.1 is 518 degrees F or higher, and the wet boiling temperature rating is 375 degrees F or higher. Some racing brake fluids exceed the dry boiling temperature rating, but may only meet the wet boiling temperature requirements for DOT 3 fluid (284 degrees).

If you want to see a detailed comparison of brake fluid performance specifications and prices by brand name, see this Brake Fluid Comparison Chart compiled by Matt Robertson. Matt researched the wet and dry boiling points for each brand listed, and even included the unit price and per ounce price so you can compare one brand of brake fluid to another.

So the next time you are inspecting or servicing your brakes, be sure to check the condition of the fluid as well as the fluid level in the master cylinder. If you add or change fluid, use type of fluid specified by the vehicle manufacturer (DOT 3 or 4) and use the highest quality fluid you can get. And above all, remember the benefits of changing the brake fluid for preventive maintenance.

brake fluid More Brake Articles:

Fixes For Common Brake Problems

Doing A Complete Brake Job

Servicing Brake Hydraulics

Bleeding Brakes

Bleeding ABS Brake Systems

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