Any time you open your anti-lock brake lines for normal maintenance such as replacing pads, adding fluid and changing hoses, you need to bleed air from your anti-lock braking system (ABS). Unusual circumstances such as a leaky hose or any other vacuum in the lines that pull air into the system can bring pockets of air into the lines that make them mushy, or even cause them to fail completely. You can bleed your ABS brake lines relatively quickly with the help of a friend.
Check with the manufacturer for a particular sequence recommended for the make and model of your automobile. You can find precise schematics for your brake lines as well as the recommendations for the order in which to proceed in your vehicle's service manual. If you do not have a service manual for your automobile, take the time to find one.
Check to determine whether the air is trapped in the system below the modulator, or whether the upper unit contains air as well. The modulator is difficult to bleed and consists of 10 or more valves. Most normal maintenance bleeding requires only bleeding the lines below the master cylinder. In that case, plan to bleed the line farthest from the master cylinder and then bleed the other line associated with that cylinder before moving to the opposite side and repeating with the other master cylinder lines. Check your service manual for the exact configuration of your vehicle.
Put the car on jack stands or, better yet, a lift, and remove all four wheels. Remove the hub caps---usually, by hand. Next, remove all lug nuts with a lug wrench. Place the lug nuts in a safe place so you can replace them later. Slide the wheel off of the hub.
Loosen the bleeder values on the brake lines with a box wrench. If you find the bolts on the bleeder valves hard to turn, spray some lubrication oil on them and let them sit for about 24 hours. Do not open them; just check to ensure you can turn the bolts at this point.
Use an old turkey baster to suck out as much of the old fluid as you can from the top of the master cylinder. Absorb any sediment remaining in the reservoir with a lint-free cloth. Keep the area as clean as possible, and avoid dripping brake fluid on painted surfaces because it will remove paint on contact.
Push one end of the clear plastic tubing over the bleeder bolt on the right rear side of the vehicle. Place the other end of the tube into a small bottle containing about an inch of brake fluid, which will prevent air from being sucked back into the system.
Fill the brake fluid reservoir with fresh brake fluid.
Place the 1-by-4-inch wood block under the brake pedal to prevent the addition of fluid from pressing it down too far.
Have your helper sit in the car. Anyone can do this, and your helper will not need to change or protect clothing.
Give your helper two simple commands: When you say "down," he must put pressure on the brake pedal; when you say "up," he releases. Explain that he should not use unusual force. Ideally, your helper should be a driver who knows what average pressure on a brake pedal feels like. Ask your helper to repeat the command you give before following the action to avoid mishaps.
Give the "down" command. Warn your helper that the brake pedal will go soft. Let her know ahead of time that when this happens she must keep pressure on the brake pedal. Give the bleeder bolt a quarter turn. Fluid will squirt out of the end of the clear tube into the bottle. When the fluid stops trickling out, tell your helper "up" to release the brake. Repeat until you see fresh fluid coming through.
Refill the brake fluid reservoir every half dozen compressions. Keep the reservoir more than half full at all times.
Tighten the bleeder bolt when clean fluid comes from the lines when compressed. Move to the next bleeder valve.