There are few things more unsettling in a car than stepping on the brake pedal and feeling it go soft and spongy. That mushy, unresponsive feeling is a classic sign that you have air in your hydraulic brake lines. This is a serious safety issue that needs to be addressed immediately, but the good news is that the fix is a straightforward procedure known as bleeding the brakes.
While it might sound intimidating, bleeding your brakes is a task that many DIY mechanics can handle with the right tools and a little patience. This guide will explain why air in the system is so dangerous and walk you through the process of how to get it out. Following these steps will help restore a firm, confident feel to your brake pedal.
Why Air in Your Brakes is a Big Problem
Your car's brake system works using a special liquid called brake fluid. This fluid is incompressible, meaning you cannot squash it. When you press the brake pedal, the fluid transfers that force perfectly to the brakes at each wheel.
Air, on the other hand, is highly compressible. If air bubbles get into your brake lines, they act like tiny springs in the system. When you press the pedal, your force is wasted on squeezing these air bubbles instead of being applied to the brakes, which is why your pedal feels soft and ineffective.
When Do You Need to Bleed Your Brakes?
Bleeding the brakes is not a part of routine, time-based maintenance like an oil change. It is a procedure that is required whenever air has been introduced into the sealed hydraulic system. There are a few common scenarios that will require you to bleed your brakes.
The most important reason is for safety. A firm brake pedal feel tells you a lot about the health of your system, and any spongy feeling should be investigated immediately.
After Replacing Brake Components
The most common reason to bleed the brakes is after you have replaced a hydraulic part. Any job that requires you to open a brake line, such as replacing a brake caliper, wheel cylinder, or rubber brake hose, will allow air to enter the system. Bleeding the brakes is the final and most critical step of any of these repairs.
If the Master Cylinder Runs Low
The brake fluid reservoir on your master cylinder has "minimum" and "maximum" fill lines. If the fluid level drops below the minimum line, the master cylinder can draw air into the system. This can happen due to a fluid leak or simply because it's time to change your brake pads after a very long time.
As Part of a Brake Fluid Flush
Brake fluid absorbs moisture over time, which lowers its boiling point and can cause internal corrosion. For this reason, it is recommended that you flush your brake fluid every two to three years. The process of flushing out the old fluid and replacing it with new fluid is the same as the process for bleeding out air.
The Tools and Supplies You Will Need
Before you start, it is important to gather all the necessary tools and supplies. Having everything ready will make the job go much more smoothly, as detailed in this checklist for your next brake job.
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A set of box-end wrenches to fit your car's bleeder screws
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A clear plastic hose that fits snugly over the bleeder screw
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A clean, clear jar or bottle to catch the old fluid
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A fresh, unopened bottle of the correct type of brake fluid for your car
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A turkey baster or syringe for removing old fluid from the reservoir
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A friend to help you pump the brake pedal
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Jack and jack stands to safely raise and support the vehicle
How to Bleed Car Brakes: The Step-by-Step Guide
The traditional and most reliable way to bleed brakes requires two people: one person to operate the brake pedal and another to open and close the bleeder screws. Always start at the wheel that is farthest from the master cylinder and work your way closer. The most common sequence is right rear, left rear, right front, and then left front.
It is absolutely critical that you do not let the master cylinder reservoir run empty during this process. If it does, you will suck more air into the system and have to start all over again. Keep a close eye on the fluid level and top it off frequently.
Preparing for the Job
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Consult Your Manual: Before you begin, check your vehicle's service manual for any specific instructions or a different bleeding sequence.
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Safely Lift the Vehicle: Use a jack to lift the car and securely support it on jack stands. Never work on a car that is only supported by a jack. Remove the wheels to get access to the brake calipers and their bleeder screws.
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Prepare the Master Cylinder: Open the car's hood and locate the brake fluid master cylinder. Clean the cap and the area around it thoroughly to prevent any dirt from falling in. Use a turkey baster to remove most of the old, dark fluid from the reservoir and refill it to the "max" line with fresh fluid.
Performing the Bleed
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Attach the Hose: Locate the bleeder screw on the first brake caliper. It is a small nipple-like valve, usually with a rubber cap on it. Place the correct size box-end wrench over the screw, then attach your clear plastic hose to the end and run the other end of the hose into your catch jar with a little clean fluid in the bottom.
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Coordinate with Your Helper: Now it is time to work with your partner. The sequence of commands is simple:
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You say, "Press the pedal." Your helper will press the brake pedal down slowly and steadily and hold it.
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As they are holding the pedal down, you will quickly open the bleeder screw about a quarter of a turn. You will see fluid and air bubbles flow out of the hose.
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Before the pedal reaches the floor, you will close the bleeder screw snugly and then say, "Release the pedal." Your helper will then let the pedal up.
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Repeat the Process: You will repeat this sequence several times on the same wheel. Keep watching the fluid coming out of the hose. Once you no longer see any air bubbles and the fluid runs clear, you are done with that wheel.
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Move to the Next Wheel: Before moving on, double-check and top off the master cylinder reservoir with fresh fluid. Then you will move to the next wheel in the sequence (usually the left rear) and repeat the entire process until all four wheels are done.
After the Job is Done
Once you have bled all four calipers, your job is almost finished. Do one final check of the master cylinder and top it off to the "max" line. Securely replace the cap.
Carefully check all four bleeder screws to make sure they are snug and not leaking. Then you can put the wheels back on, lower the car, and torque the lug nuts to the proper specification. The final step is to test your work with a slow, careful test drive.
A Firm Pedal and Peace of Mind
Bleeding your brakes is a fundamental piece of automotive maintenance that is directly tied to your safety. By removing all the air from the system, you ensure that your brake pedal is firm and that your car has the full stopping power it was designed with. It is a simple process that pays huge dividends in confidence and peace of mind.
At NRS Brakes, we believe that safety is the most important part of any brake system, which is why we produce a variety of premium brake products. From the hydraulic fluid to the brake pads themselves, every part must work perfectly. We engineer Brake Pads with our patented mechanical attachment technology to guarantee that the friction material will never separate, giving you one less thing to worry about on the road.
What is one DIY car maintenance task that you have found to be especially rewarding?
