Brake fluid does wear out and become contaminated.

For years, many people considered brake fluid to be a brake-job-to-brake-job service item. Do a brake job, and rebuild the wheel cylinders. Then, bleeding the brakes will flush the system – sort of.

Well, brake fluid always was, and continues to be hygroscopic – meaning that it absorbs moisture like a sponge. Leave a container of brake fluid open for a few hot, humid summer days, and it will likely absorb enough moisture to render it un-useable according to DOT specifications.

From a brake engineering point of view, absorbing and dispersing moisture throughout the system is good. It keeps moisture from forming in puddles within the system. Of course, these water puddles would settle into the lowest areas – such as wheel cylinders and calipers – where they can easily be boiled into steam by the heat generated by hard braking conditions and cause a loss of braking known as brake fade.

The downside of brake fluid being so hygroscopic is that the moisture dispersed throughout the system lowers the brake fluid boiling point, increases its viscosity, and promotes rust and corrosion.


The DOT 3 specification requires a minimum boiling point of 401 Fahrenheit for “dry” brake fluid – fluid that contains no moisture whatsoever. To meet DOT 3 specs., “wet” or fully saturated fluid should reach 284F before boiling. (That’s only 140 C or Gas mark 4)

DOT 4 specs calls for minimum boiling points of 446F dry and 311F wet.

Most DOT 3 and DOT 4 brake fluids sold today meet or exceed the above specifications, but it is important to know the numbers.

NAPA DOT 3 brake fluids, for example, actually exceed minimum DOT 4 standards with a dry boiling point of 450, while many of the cheaper brake fluids barely meet the minimum requirements to call themselves a DOT 3.

As with most other things there really IS a difference between a cheap brake fluid and a quality brake fluid.

DOT 5 is silicone-based brake fluid with a wet boiling point of more than 500F.

You may ask yourself why the manufacturers do not simply switch to DOT 5 spec fluid. After all if some is good, more is better, and too much is just right. Right ?

Well the DOT 5, while better under heat conditions than DOT 3 or DOT 4, does NOT absorb any moisture, so any moisture that finds its way into the system WILL puddle up somewhere causing rust, corrosion and, possibly, brake fade. You don’t want slugs of water floating around any brake system.

Also, DOT 5 silicone fluid contains more absorbed air than DOT 3 or DOT 4 glycol, and it will aerate much more easily when it is pumped through small orifices such as those on an ABS system. For that reason, most manufacturers warn against using a DOT 5 fluid in cars with ABS.

DOT 5 is recommended for vehicles that sit for long periods such as antiques and classics that are stored all winter, and, of course, for racing due to its high heat tolerance. But conditions within the hydraulic system MUST be monitored.


Even in sealed automotive brake systems, brake fluid will absorb one percent or more moisture every year simply from opening the reservoir to check fluid level, opening the bleeders, and through microscopic pores in the rubber hoses.

According to one trade publication we read, many two-year-old vehicles have been found to contain two to three percent water in the brake fluid. Imagine how much water must be contained in some six, seven, eight year or older vehicles that have never had their brake fluid serviced.

One percent water content can push DOT 3 fluid down to a 369C boiling point while two percent can push it down to 320C. Three percent can get it down to 293C – dangerously close to minimum DOT 3 requirements.

The rate at which this occurs depends on a lot of things such as the age of the vehicle and the type of hoses used. Better quality hoses are lined to make them less permeable to moisture. Some manufacturers chose to skip the lining in order to sell their product a little cheaper. You get what you pay for !

DOT (Department of Transportation) 3 and DOT 4 specification brake fluids are made from glycol and additives. Glycol absorbs and disperses moisture. The additive package helps to keep the moisture from attacking the internal components of the brake hydraulic system.

Another area of concern on today’s automobile that was not a factor years ago is Antilock Braking Systems or ABS. There are many close tolerance components within the ABS system, and moisture can wreak havoc with them. Replacing a single ABS component is often much more expensive than a simple brake fluid service would have been.  

The average car on the road today is 10 years-old. According to Brake & Front End Magazine, only half of these cars have ever had their brake fluid changed.

For the average motorist, there might not be that much risk under normal driving conditions. But, prolonged braking such as mountain driving and trailer towing might tax old brake fluid beyond its capacity.