Rick's Free Auto Repair Advice

DOT 4 Brake Fluid

DOT 4 Brake Fluid

What is DOT 4 Brake Fluid

DOT 4 brake fluid is a brake fluid that complies with the standards established by the Society of Automotive Engineers (SAE) and the U.S. Department of Transportation (DOT). To comply, the fluid must meet these criteria:

• The fluid must have a stable viscosity and be free-flowing at all operating temperatures.
• DOT 4 brake fluid has a DRY boiling point of 446°F and a wet boiling point of 311°F.
• Must be non-corrosive to all the metal, rubber, and composite materials used in the brake system.
• The anti-corrosion additives must prevent corrosion or rust formation in the system.
• The fluid must have lubricating properties to inhibit wear to seals and other moving parts.
• The fluid must have very low compressibility.

DOT 4 Dry versus wet boiling points

Glycol-based brake fluids are hygroscopic, meaning they have an affinity for water. Even though the brake system is thought to be a “closed” system, moisture can still get in. First, every time a shop or driver opens the master cylinder reservoir, the fluid is exposed to moisture in the air. However, even if you never open the master cylinder, moisture can still get in. As the caliper piston and wheel cylinder pucks move in and out of the bore, pores in the metal can carry moisture into the brake system. Moisture can also penetrate through flexible brake hoses and seals.

Brake materials stop the vehicle by transforming motion into friction and then into heat. That heat can cause the moisture in brake fluid to boil and turn into steam. Since steam is compressible, moisture in the system can greatly reduce braking ability, causing the brake pedal to feel “spongy.”

Studies show that braking performance can drop significantly with just 3% water absorption. In fact, if the fluid contains 3% water, the boiling point drops to 212⁰F.

• The DRY boiling point specification is for fresh, uncontaminated fluid with no water absorption.

• The wet boiling point, is the point at which brake fluid will boil with a specific amount of water absorption. The technical term for this standard is the Equilibrium Reflux Boiling Point (ERBP).

How to measure brake fluid moisture

Use a refractometer or a battery-powered tester

Brake fluid moisture tester to determine condition of brake fluid in car or truck

Battery-powered brake fluid moisture tester

how to test antifreeze, coolant


How to test brake fluid condition

Contrary to popular belief, color is NOT a good indication of brake fluid condition. You must check the fluid’s moisture level AND the fluid’s copper content. Metal brake lines are formed from flat steel and rolled into the tubing. The seam is brazed. As the anti-corrosion additives wear out, the brazing deteriorates and shows up in the fluid as copper. Test strips are used to measure the copper content. When the copper content exceeds the test values, OR the fluid’s moisture content exceeds 2-3%, the fluid must be flushed to maintain proper braking and prevent further corrosion.

brake fluid test strip

Brake fluid test strips test the state of anti-corrosion additives in the brake fluid by testing for copper content

DOT 3 versus DOT 4 viscosity

DOT 3 has a higher viscosity than DOT 4, meaning the two fluids are NOT interchangeable unless specified by the carmaker. Even though DOT 4 has a higher boiling point, which may lead you to think it’s a “better” fluid, its lower viscosity may not be right for your particular vehicle.

Always install the recommended brake fluid type.

©, 2018 Rick Muscoplat


Posted on by Rick Muscoplat


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