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Brake Fluid

Are DOT 3 and 4 good enough?

Like every race driver, I’ve experienced the heart-stopping feeling of failing brakes. When you’re going 145 and the next corner can be negotiated no faster than 47 mph, a racer must know the brakes will work. When facing fading brakes during a race, I boost my confidence by tapping the brakes lightly with my left foot on the straightaway while keeping my right planted to the floor: Not only does it let me know if the brakes are going to be home when I need them, it psyches out the drivers behind me. I’ve had brakes fail because the pad material completely wore away, brake calipers snapped clean off, and the brake fluid boiled. While driving an 80,000-pound 18-wheeler, I’ve had the beginning of bake fade on a mountainous downhill grade: That’s no fun either. And, back in the days of drum brakes, I’ve experienced brake failure of all types.

The Importance of Brake Fluid

The primary function of brake fluid is to transfer the force the driver exerts on the brake pedal to squeeze the brake pads against the rotor. The resulting friction slows the car, if everything works correctly.

Thanks to advances in brakes, brake pads and brake fluid, few drivers of passenger vehicle drivers must deal with brake fade or failure. But don’t get complacent: cheap or aging brake fluid can raise your heart rate quicker than anything shy of a mugger’s pistol.

In many brake fade or failure situations, the brake pads are not the problem but rather it’s the brake fluid. If you’re descending a steep grade with a loaded SUV, playing race car driver on a canyon road or attempting to stop for an unexpected interstate jam, the brake discs and pads may reach or exceed 800 degrees Fahrenheit. Brake design keeps much of that heat away from the brake fluid. But if you don’t allow the brakes to cool between applications, lower-quality or old brake fluid can overheat and cause serious problems.

If your fluid is old it’s full of water. As brake fluid ages it attracts water: engineers call it hydroscopic. That moisture boils at 212 degrees and creates pockets of air in your brake lines. Also, bits of rubber from deteriorating seals and other contaminates enter the system. These prevent the brake pads from pressing against the discs as hard as necessary. The moisture, dirt and other contaminates also cause premature failure of internal brake components.

The U.S. Department of Transportation has set boiling-point standards for brake fluids. The most common brake fluids are DOT 3 and DOT 4. Both are usually polyethylene glycol-based. Brand new DOT 3 boils at about 400 degrees. Top quality DOT 4 won’t boil until more than 500 degrees, though the legal minimum is about 450 degrees. That’s more than plenty unless you’re racing.

But add about three percent water—which is what the government does for testing—and the boiling point drops to less than 285 degrees for DOT 3, while DOT 4 falls at barely over 310 degrees. That’s cutting it close if you’re descending Independence Pass outside Aspen, Colo. with a loaded trailer.

If your car came with DOT 3, it’s okay to upgrade to DOT 4, though DOT 3 is just fine for the vast majority of drivers. But if your car came with DOT 4, use only DOT 4 fluid. Both DOT 3 or DOT 4 work fine with most disc or drum brakes and anti-lock braking systems (ABS).

There are a couple of other grades of brake fluid: DOT 5 and DOT 5.1. DOT 5 is silicone based and offers extremely high boiling points. However, DOT 5 doesn’t play well with DOT 3, 4 or 5.1, as well as some original-equipment brake system components. Consider it a track-racing-only fluid.

Synthetic, non-silicone brake fluids—called DOT 5.1—offer boiling points (500 dry and 356 wet) exceeding DOT 5, and are fully compatible with DOT 3 and DOT 4 fluids.

What this means for the do-it-yourselfer:

- Top quality brake fluid is cheap insurance;

- If you open a brake fluid container, put the fluid in the car immediately, or discard according to recycling rules;

- Clean off the area around the brake fluid reservoir before adding fluid;

- Regularly check the brake fluid level. As brake pads wear, more brake fluid must be added;

- If you notice a “spongy” feeling pedal, you’re got a problem that won’t cure itself;

- Rapidly dropping brake fluid level (or brake fluid puddles under your car) signals a serious problem and should be addressed immediately;

- Renewing brake fluid (mechanics call it flushing) every couple of years can extended life of internal brake components and improve emergency stopping performance.

Brake fluid: Few think about it until it’s far too late. Separate yourself from the crowd.

About the Author

Mac Demere is a vehicle tester and race driver who competed in the NASCAR Southwest Tour and Daytona 24 Hours.