DOT 3 vs. DOT 4 Brake Fluid: What’s the Difference?

It’s tough to avoid feeling overwhelmed by the amount of choices when shopping for fluids for your vehicle.

Over my career as a certified automotive technician and instructor, I’ve repaired numerous brake systems damaged by incorrect, contaminated or worn-out brake fluid. The worst and most expensive happened when a customer added power steering fluid instead of brake fluid to the master cylinder. Every non-metallic brake system part needed replacing.

His explanation? “Well, it worked for MacGyver.”

Any damage to the brake mechanical or hydraulic system — most important for safety — can result in your brakes failing. The more you know about the fluid that keeps your brakes working, the better.

What Is Brake Fluid?

A solvent-based, hydraulic fluid containing synthetic additives (lubricants, corrosion inhibitors and stabilizers) that extend the life of your fluids and brake parts.

Stepping on the brake pedal forces pressurized brake fluid into disc brake calipers, causing the brake pads to clamp down onto the disc rotors. This creates tremendous heat that brings your car to a halt.

What Does the DOT in DOT Brake Fluid Stand For?

The U.S. Department of Transportation (DOT). The DOT sets the standards for brake fluids, including:

• Chemical interactions with the materials (rubber, composites, and various metals) used in braking systems;
• Containers accurately labeled, color-coded and sealed to prevent contamination.
• Viscosity levels;
• Chemical, corrosion and temperature stability;
• Boiling points.

Importance of brake fluid boiling points

Brake fluid is extremely hygroscopic, meaning it can absorb and hold moisture — usually 2% a year by volume.

Unfortunately, the heat friction that slows rotor and wheel rotation, stopping your car, transfers to the brake fluid. This causes the moisture to boil, producing gas bubbles. Gas bubbles in brake fluid can compress, causing brake fade (the pedal feeling mushy) and greatly increasing stopping distances.

Most importantly, the DOT classifies brake fluid based on dry and wet boiling points. A higher boiling point reduces the chance of gas bubbles forming.

• The dry boiling point is the temperature where brake fluid from a new, unopened sealed container begins to form gas bubbles.

• Wet boiling point is the temperature where brake fluid, which has absorbed 3.7% of water by volume, begins to form gas bubbles.

Moisture in brake fluid also causes metal brake parts to rust and corrode from the inside out (aka a rotted-through brake line), and rubber parts to swell, deteriorate and weaken.

What Is DOT 3 Brake Fluid?

DOT 3 brake fluids are glycol ether-based solvents with much higher boiling points than mineral-based DOT 2. DOT 3 was introduced in the 1960s, and today remains the standard used in most vehicles.

What Is DOT 4 Brake Fluid?

DOT 4 brake fluids, introduced in 2006, are similar to DOT 3. But they contain 20 to 30 percent borate ester, which significantly increases DOT 4 boiling points.

What’s the Difference Between DOT 3 and DOT 4 Brake Fluid?

Although there’s no standard formula for brake fluids — each brand may be composed of different chemical compounds —DOT 3 and DOT 4 brake fluids share the same excellent and not-so-great characteristics.

They do an outstanding job keeping hydraulic brake systems clean, working properly and effectively. But if spilled, they will quickly ruin your vehicle’s paint.

• DOT 3 has lower wet and dry boiling points than DOT 4, making it less suited for towing a trailer.
• DOT 4’s higher wet and dry boiling points make it more suitable for more demanding applications, like high-performance vehicles, frequent stop-and-go city driving, larger vehicles that do heavy breaking, and exotic aftermarket braking systems that operate at extremely high temperatures.
• DOT 4’s higher wet and dry boiling points make it less likely to absorb moisture. However, DOT 3 absorbs less moisture than DOT 4 and needs less frequent service. Higher boiling points mean any moisture absorbed by DOT 4 cannot disburse. This causes the additives in DOT 4 to deplete and break down quicker than those in DOT 3.

Can You Mix DOT 3 and DOT 4 Brake Fluid?

It depends. Because of its lower boiling points, never add DOT 3 brake fluid to a system designed for DOT 4.

However, because DOT 3 and DOT 4 are both glycol ether-based, you can mix DOT 4 with DOT 3 brake fluid. The correct type of brake fluid for your vehicle is usually printed on the brake master cylinder reservoir fill cap and listed in the owner’s manual.

When Should Bake Fluid Be Flushed?

Brake fluid should be flushed every two years or 30,000 miles.

It should be clear yellowish to light brown with a slick oily feel. If the fluid is dark, has black grit, or the fill cap gasket/seal is mushy, it’s time to flush the brake fluid and replace it, along with the master cylinder gasket/seal.