Brake bleeding is the procedure performed on hydraulic brake systems whereby the brake lines (the pipes and hoses containing the brake fluid) are purged of any air bubbles. This is necessary because, while the brake fluid is an incompressible liquid, air bubbles are compressible gas and their presence in the brake system greatly reduces the hydraulic pressure that can be developed within the system.
Here’s what happened. The pads wore so thin that the brake fluid level dropped too low in the master cylinder reservoir. An air bubble or three got pumped into the lines. And because air is compressible, you now have the equivalent of a very soft spring in the solid column of brake fluid between your foot and the wheels. Bleeding the brakes will flush that air out.
Aside from a severely worn brake which is the most common culprit, other causes of you having air inside your brake system include possible for a leak in the brake line to let air sneak into the brake system. Poor driving such as constantly slamming on the brakes can also lead to air in brake line.
So even after you have just changed your pads, the pedal still feels so spongy and low or a brake pedal that goes almost to the floor before engaging; you must bleed the brake.
Bleeding a brake line can be difficult and should be left to a professional. The following is a quick rundown of the steps involved when bleeding brakes:
- The brake bleed screw behind each brake is loosened and then tightened again, but not super tight. Special bleeder wrenches are required to loosen these screws.
- A flexible rubber hose will be placed over the end of the bleeder screw and the other end of the hose will be put in a jar. The jar will be filled with brake fluid to cover the end of the hose.
- A second person will pump the brake pedal a few times and then hold the brake pedal down while the bleeder screw is opened again.
- Brake fluid will squirt out and air bubbles will be visible in the fluid. While the brake pedal is still depressed the bleeder screws will be retightened. The brake pedal will now be released.
- This process will be repeated until no air bubbles are visible in the fluid. The entire process will then be repeated on each wheel.
Bleeding the brakes falls under the routine maintenance category, and should be performed over the life of a vehicle. Most experts recommend bleeding your brakes every 2 to 3 years to keep them in tip-top shape.
Technicians should avoid letting air into the actuator of ABS, EBD, BA, or other sophisticated brake systems while bleeding a brake