Every few years or so, you need to bleed the brakes on your car. This process ensures your brakes are able to run at peak performance. Bleeding the brakes involves replacing the brake fluid in the system to remove any air that may have leaked into the system. It also gets rid of old brake fluid that often becomes infused with water.
How does air get into the brake system?
Air can find its way into the braking system in a variety of ways. If you have serviced the brakes in any capacity, whether by adjusting calipers or replacing brake pads, air can leak into the brake lines. If there is a leak in the lines, fluid can get out and air can also get in. Worn pads and slamming on the brakes can also add up to gas in the line. The most common way air gets in is due to high temperatures. When the brake fluid reaches a certain point, it begins to boil. Since there is often water mixed in with the fluid, this turns to steam and eventually into air.
What happens when there's air in the lines?
Having air in the braking system will result in a soft or spongy brake pedal. The air in the lines is being compressed sooner than the liquid, so there is not as much hydraulic pressure. You may notice you have to depress the pedal further when you want to stop. You should not generally need, let alone be able to, push the pedal down more than about halfway to the floor.
How do you remove the air?
Bleeding the brakes involves slowly removing all the old brake fluid, starting with liquid in the reservoir. Once that has been removed and the tank cleaned of debris, the car will need to be jacked up. Some vehicles may need to have the wheels removed to get to the bleeder valves. Once they are accessible, you can open them and have someone else press down gently on the brake pedal to drain the fluid in the system. Each wheel must be drained separately.
After most of the old fluid has been removed, you can add new, clean liquid to the reservoir and repeat the process until the fluid coming from the valves is clean. Then top off the reservoir tank and you're done.