Why and How to Change Brake Fluid

brake fluid reservoir
When Should You Change Brake Fluid?. https://www.gettyimages.com/license/654925470

Your brakes are arguably the single most important piece of equipment on your car, and a faulty brake system quickly puts you and others in danger.

While it seems obvious that brake pads, brake rotors, and brake calipers should be maintained, brake fluid maintenance seems to be altogether forgotten—many owner’s manuals stop at checking and adjusting brake fluid level. Below we cover whether and how often brake fluid should be changed, and for do-it-yourselfers, we'll cover the hows as well.

01
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How Does Brake Fluid Work?

diagram of brake system operation
Brake Fluid is What Makes the Brake System Work. https://www.gettyimages.com/license/667043452

The brake system is made up of levers, pistons, and hydraulic fluid (brake fluid), designed to transmit brake pedal force to the four brakes. When you step on the brake pedal, small pistons in the brake master cylinder convert mechanical force into hydraulic pressure. Because brake fluid is incompressible, it transmits this pressure equally to the brakes.

The brake caliper pistons convert this hydraulic pressure back into mechanical force. Because the brake caliper pistons are larger than the brake master cylinder piston, it multiplies your force by many times to compress the brake pads.

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02
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Why and How Often Do You Need to Change Brake Fluid?

brake rotor heated up during racing
Hot Brakes Can Reveal Neglected Brake Fluid. https://www.gettyimages.com/license/187063298

Brake fluid is so overlooked that about half of all American cars and trucks over ten years of age have never had a brake fluid change. Interestingly, in Europe, where brake fluid inspection is required, about half of them fail the test.

Why do vehicles fail this test? It all has to do with a special property of brake fluid, one that prevents even bigger problems.

Brake fluid is hygroscopic, absorbing water which could easily boil at the high temperatures in the brake system. This is important, because the entire function of the brake system is to convert the kinetic energy of your vehicle into heat energy.

While water is incompressible, it boils at just 212 °F (100 °C) becoming an easily-compressible water vapor. Under normal driving conditions, brakes might reach 100 °F to 200 °F (38 °C to 93 °C), and it’s perfectly normal for the brakes to exceed 400 °F (204 °C) braking on hills.

  • Dry DOT 3 brake fluid boils at 401 °F (205 °C), which is fine in normal traffic, but might cause problems on hills, so some prefer DOT 4 brake fluid, boiling at 446 °F (230 °C).
  • Wet DOT 3 brake fluid, containing 3.7% water, boils at just 284 °F (140 °C), while wet DOT 4 brake fluid boils at 311 °F (155 °C).

The longer one waits to change brake fluid, the more water it absorbs, increasing the chance of brake fade at the worst possible moment.

You should change brake fluid about every 20,000 miles or two years.

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03
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What You'll Need to to Change Brake Fluid

brake caliper bleeder screw
This Brake Bleeder Seems Clean, but Yours Might be Rusted. https://www.gettyimages.com/license/636041498

In order to change brake fluid, you'll need the following. Note that if you've ever "bled" your brakes to address brake pedal sponginess (an indication compressible air has gotten in) then you already know how to change brake fluid.

You’ll need:

  • Jack and Jack Stands – To lift and support your vehicle.
  • Wheel Removal Tools – To remove your wheels to access your brakes.
  • Torque Wrench – To reinstall your wheels.
  • Brake Bleeder Tool – Buy one or make one of a plastic bottle and hose.
  • Brake Bleeder Wrench – Usually an 8 mm or 10 mm box wrench, to loosen the bleeder screw.
  • 32 oz Brake Fluid – You’re going to use a lot to flush out the old fluid.
  • New Bleeder Caps – Bleeder caps degrade or get lost.
  • Brake Fluid Siphon – To remove old brake fluid from the brake master cylinder reservoir.
  • Brake Cleaner and Rags – To clean everything up.
  • PPE – Nitrile gloves and safety glasses, as brake fluid is an irritant.
  • Rust Penetrant – WD-40 or PB Blaster can help loosen an old bleeder screw.
04
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Step by Step Brake Fluid Change

a brake bleeder bottle
A Brake Bleeder Bottle is an Easy Tool to Make. https://www.gettyimages.com/license/511509585

Start by lifting and supporting your car on jack stands and removing the wheels.

Remove the bleeder caps and spray the bleeder screws with rust penetrant. While this is working in, open the hood and remove the master cylinder reservoir cap.

Use the siphon or extractor to remove as much of the old brake fluid as possible. You might need to remove a strainer to get deeper into the reservoir. Refill the reservoir, then move on to bleed each wheel in order, right rear (RR), left rear (LR), right front (RF), left front (LF). Important: Do not let the reservoir go empty, otherwise you must start over to get air out of the master cylinder.

  1. Place the bleeder wrench on the bleeder screw, then attach the plastic hose. Open the bleeder 1/4-turn and pump the brake pedal 5 or 6 times. Check and refill the brake fluid level in the master cylinder reservoir.
  2. Pump the brake pedal another 5 or 6 times. Check for fresh fluid and no bubbles in the bleeder hose. If fluid is still dark, another 5 or 6 pumps might be needed to finish the job. Aim to pump about 8 oz of new brake fluid into the system for each brake, then close the bleeder screw.
  3. Repeat A and B for the LR, RF, and LF brakes.
  4. After checking all brake bleeders are closed, fill the master cylinder reservoir to “FULL,” install the cap, and start the car. Step on the brake pedal and check that it feels firm. Clean any spilled brake fluid, install the bleeder caps, install the wheels, torque the wheel nuts, and go for a test drive. Used brake fluid can be recycled with your used oil.

Now, to change brake fluid might sound like a lot of steps, but it’s a simple job that can significantly improve braking effectiveness and vehicle safety.