The automotive braking system is a hydraulic system which includes a non-compressible fluid to transfer braking force from your foot to the working components fixed to the vehicle's wheels. When these systems are serviced, air can enter through an open line. Air can also enter the system through a leaking fluid line. The compressible air entering the system or a fluid leak can seriously hinder braking performance and the air must be purged from the system once it is repaired. This can be accomplished through bleeding or purging the air from the brake lines and this guide will aid in the process.
The process of bleeding the brake system resembles flushing the brake fluid. When the brakes are being bled, the intent is to remove any trapped air from the system. A brake fluid flush service serves to completely remove any old fluid and contaminants.
Part 1 of 2: Brake System Concerns
The typical symptoms experienced when there is a fluid leak usually include:
- The brake pedal drops to the floor and often will not return.
- The brake pedal may become soft or spongy feeling.
Air can enter the hydraulic braking system through a leak which must be repaired before attempting to bleed the system. Weak wheel cylinder seals in drum brakes can start to leak over time.
If you live in an area where salt is used regularly to de-ice roads because of cold weather, rust can develop on exposed brake lines and rust through them. It would be best to replace all of the brakes lines on this vehicle, but some kits allow the lines to be replaced in sections.
Many modern vehicles with anti-lock braking (ABS) systems require the system module to be bled through a special procedure that often requires the use of a scan tool. If this is your case, hire a qualified professional as air bubbles can become trapped in these units and be very difficult to purge.
- Note: Check your vehicle service manual and take a look under the hood for a master cylinder or ABS module that may have a bleeder screw on it. Start at the wheels and work your way back to the master cylinder for the best results if you cannot find the specific procedure.
Other hydraulic brake system concerns are:
- A stuck brake caliper (caliper may be stuck clamped or released)
- Clogged flexible brake hose
- Faulty master cylinder
- Loose drum brake adjustment
- Leaking fluid line or valve
- Failed/leaking wheel cylinder
These failures can result in the replacement of a component and/or require the brake fluid system to be purged of air and the fluid flushed. If you notice a soft, low or spongy pedal along with increased braking effort, it is important that you seek service immediately.
Part 2 of 2: Bleeding the brakes
This brake fluid purging method will allow for the process to be done without a partner. Be sure to use the right fluid to avoid contamination of the brake fluid and damage the braking system.
Offset head designs work best and must include at least ¼,⅜,8mm,and 10mm sizes. Use the wrench that fits your vehicle's bleeder screws.
- Clear tubing (12” long section sized to fit tight over your car's bleeder screws)
- Brake fluid
- Can of brake cleaner
- Disposable bottle for waste fluid
- Floor jack
- Jack stand
- Rag or towel
- Socket to fit your lug nuts (1/2")
- Torque Wrench (1/2")
- Vehicle service manual
- Wheel chocks
Tip: 1 pint of brake fluid is usually enough for bleeding and 3+ will be required if a major component is being replaced.
Step 1: Set the parking brake. Set the parking brake and place a wheel chock at each wheel.
Step 2: Loosen the wheels. Loosen the lug nuts on all wheels about a half turn and prepare your lifting equipment.
Tip: The service can be done one wheel at a time or the entire vehicle may be lifted and supported on jack stands as long as the vehicle is on a level surface. Use common sense and create a safe work environment.
Warning: Some vehicles have a bleeder screw on the ABS module and the master cylinder. See vehicle service manual for more detailed information.
Step 3: Open the hood and inspect the current level of the brake fluid. You can use the Max and Min markings for a reference. You do not want the brake fluid level to ever go below the minimum level marking.
- Tip: On some brake fluid reservoir designs, you can use a turkey baster or syringe to remove old fluid to make the fluid flush process a little faster.
Step 4: Fill the brake fluid reservoir to the Max mark. More can be added, but be sure not to spill brake fluid. Brake fluid can eat away protective coatings that prevent rust and make a big mess.
Step 5: Check the bleeding sequence for your vehicle in the service manual. Start where the service manual recommends or your can usually begin at the bleeder screw furthest from the master cylinder. This is the right rear wheel for many vehicles and you continue with the left rear, right front, then bleed the left front braking assembly.
Step 6: Lift the corner of the vehicle where you will begin. Once the corner is lifted, place a jack stand under the vehicle to support the weight. Do not crawl beneath a vehicle that is not supported by the proper equipment.
Step 7: Remove the first wheel in the sequence. Locate the bleeder screw at the rear of the caliper or drum brake wheel cylinder**. Remove the rubber cap from the bleeder screw and don't lose it. These caps keep out dust and moisture that can rust the bleeder opening shut.
Step 8: Place the box-end wrench over the bleeder screw. An offset wrench works best because it allows the most room for movement.
Step 9: Place one end of the clear plastic hose over the nipple of the bleeder screw. The hose section should fit snug over the nipple on the bleeder screw to prevent an air leak.
- Warning: The hose must be able to stay on the bleeder screw to prevent air from being sucked into the brake lines.
Step 10: Place the other end of the hose into the disposable bottle. Place the discharge end of the clear hose into a disposable bottle. Insert a long enough section so the hose will not fall out and make a mess.
- Tip: Route the hose in a way where the hose rises up above the bleeder screw before curving back down to the container or mount the container higher than the bleeder screw. This way gravity will allow the fluid to settle while the air rises away from the fluid.
Step 11: Use the wrench to loosen the bleeder screw about ¼ turn. Loosen the bleeder screw while the hose is still attached. This will open the brake line and allow fluid to flow.
- Tip: Since the brake fluid reservoir is higher than the bleeder screws, the effects of gravity may cause a small amount of fluid to flow into the hose when the bleeder screw is opened. This is a good sign there aren’t any blockages in the fluid line.
Step 12: Apply the brake pedal slowly, two times. Go back to the brake assembly and inspect your tools. Make sure fluid is traveling into the clear tubing and the fluid is not leaking from the tube. There should be no leaks as the fluid makes it’s way to the container.
Step 13: Apply the brake pedal fully and slowly 3-5 times. This will force fluid from the reservoir, through the brake lines and out of the open bleeder screw.
Step 14: Check to make sure the hose has not slipped off of the bleeder. Make sure the hose is still on the bleeder screw and all fluid is contained in the clear hose. If there any leaks, air will be drawn into the braking system and more bleeding will be required. Inspect the fluid within the clear hose for air bubbles.
Step 15: Check the brake fluid level in the reservoir. You will notice the level has dropped a bit. Add more brake fluid to fill the reservoir again. Do not let the brake fluid reservoir run dry.
- Note: If there are air bubbles present in the old fluid, repeat steps 13-15 until the fluid is clean and clear.
Step 16: Close the bleeder screw. Close the bleeder screw before removing the clear hose to prevent air from being drawn in. The bleeder screw does not take a lot of force to close. A short tug should do the trick. Brake fluid will spill from the hose so have a rag ready. Spray a little brake cleaner to rid the area of brake fluid and replace the rubber dust cover.
- Tip: Close the bleeder screw and take this time to go back into the car and step on the brake pedal again. Notice the feel. If the pedal was soft before, you should feel the pedal become stiffer as each component is purged.
Step 17: Check to make sure the bleeder screw is tight. Replace the wheel and spin on the lug nuts as a note you have completed the service on this corner. if you are servicing one corner at a time. Otherwise, continue to the next wheel in the bleeding sequence.
Step 18: Next wheel, repeat steps 7-17. Once you have access to the next corner in the sequence repeat the bleeding process. Be sure to keep a watchful eye on the brake fluid level. The reservoir must remain full.
Step 19: Clean any residual fluid. When all four corners have been bled, spray the bleeder screw and any other parts moistened with spilled or dripped brake fluid with brake cleaner and wipe dry with a clean rag. Leaving the area clean and dry will make it easier to spot leaks. Try to avoid spraying the brake cleaner on any parts made of rubber or plastic, as the cleaner can make these parts brittle after time.
Step 20: Test the brake pedal for a firm feel. Bleeding or flushing the brake fluid will generally improve the pedal feel as the compressible air is removed from the system.
Step 21: Inspect the bleeder screws and other fittings for signs of leakage. Correct as necessary. If a bleeder screw had been left too loose, you must begin the entire process over again.
Step 22: Tighten all wheels to factory specification. Support the weight of the corner you are tightening down with the floor jack. The vehicle can be raised, but the tire must be touching the ground or it will just spin. Use the ½” torque wrench and lug nut socket to properly fasten the wheel. Torque each lug nut before removing the jack stand and lowering the corner. Continue to the next wheel until all are fastened.
- Warning: Properly dispose of the used waste fluid as you would dispose of used motor oil. Used brake fluid should NEVER be poured back into the brake fluid reservoir.
This one person method is very effective and will offer a great reduction in moisture and air trapped in the hydraulic braking system as well as provide a very firm brake pedal. Time for a test run. Step firmly on the brake pedal to be sure it is nice and firm before starting the vehicle. At this point, it should feel almost like stepping on a stone.
You may feel the pedal sink or rise as the vehicle starts and the power brake assist starts to kick in. This is normal as the power brake assist system amplifies the force you apply with your foot and sends all of that force through the hydraulic system. Take the vehicle for a drive and slow the vehicle with firm brake pedal applications to test your work. The brakes should have a very quick and sharp response to your pedal applications. If you feel the pedal is still too soft or braking performance is lacking, consider hiring one of our mobile experts here at YourMechanic for assistance.