Story and Photography by Ryan Lee Price
One of the most important steps in making the brakes safe and reliable for everyday use is to properly bleed the system. The concept behind bleeding a brake system is to remove any air bubbles left in the lines. It is essential to bleed the brakes before operating the vehicle if you replace the master cylinder, wheel cylinders, or any of the brake lines or if you are introducing fluid into the system.Brake fluid, much like water, cannot be compressed; which is why a hydraulic braking system is so effective and efficient.First off, how does a simple dual-circuit hydraulic foot-brake system work? When you step down on the brake pedal and it feels soft, spongy or weak, odds are good that air is in the system (or fluid pressure is low). Even though we call the brakes a hydraulic system, it doesn’t use water, but instead, uses a brake fluid with properties similar to water. Like water, brake fluid is incompressible, meaning that it will never take up less space under pressure (only one gallon of fluid will fit into a one-gallon jug), whereas air under pressure compresses into a smaller volume. Apply this principle to the brake system and when a bubble of air is stuck in the lines, the air bubble compresses instead of passing along the force of the pedal to the wheel cylinders via the master cylinder. As you press down on the pedal, you actuate the master cylinder which increases pressure in the lines. In a typical system, the master cylinder has two pistons, one for the front and one for the back brakes. Hydraulic pressure allows the wheel cylinders (one located at each wheel) to squeeze the brake shoes against the drums (or discs). The reservoir refreshes the system, and as you release the brake pedal the pistons in the master cylinder relax, and the fluid returns to its original position.
Of course, bleeding the brakes isn’t a salve for all of the problems with any brake system, but if bleeding the system doesn’t fix a weak pedal, there is probably a leak somewhere. Since brake systems vary, let’s demonstrate the concept of bleeding the braking system with a simple classic Volkswagen, a 1971 Super Beetle. It is easier to access the wheels and the system if you jack the car up and safely rest it on jack stands. Keep it on level ground, as well. This way, judging the reservoir level is more accurate.If the reservoir is full, but the liquid inside is questionable (dirty or littered with debris), make sure to flush the system before disconnecting it or making repairs. Be careful, as brake fluid is highly corrosive and will turn a beautiful paint job into bare metal in moments.Start by filling the reservoir until it is approximately one-half inch from the bottom of the neck, and during the bleeding process check the level after each wheel. If it goes dry, you’ll have to refill and start all over again. Below the bolt for the wheel cylinder and above the hard brake line is the bleeder valve, which may be different on different models. On the Volkswagen, it is covered by a small rubber grommet and kept secure by a 7mm bolt. There’s no need to purchase a fancy brake bleeding kit, as an old soda bottle with a length of fuel line fed through the cap and into a small amount of brake fluid will work well. Make sure the fuel line fits snugly over the bleeder valve.Just as brake lines can contain air, air can also be present in the master cylinder. Loosening the fitting for the master cylinder rear circuit and having an assistant pump the pedal a couple of times forces the air bubble. After a few pumps, have the assistant hold the pedal down while you tighten the fitting.
After clearing the master cylinder, start with the wheel farthest from the master cylinder, which would be the rear passenger wheel. Attach the reserve bottle to the bleeder valve and loosen the 7mm bolt one-half turn. While the valve is open, have the assistant slowly depress the brake pedal and hold it. Watch the soda bottle for bubbles. When the bubbles stop, close the valve. Repeat this process a couple of times for each wheel, or until the bubbles stop.
Complete this same process on all four wheels, working your way closer to the master cylinder. In addition to pumping the brakes, the assistant can let his or her foot slip off of the brake pedal, forcing a sudden surge of fluid into the lines ... and the air out.The idea behind dipping the other end of the bleeder line in the fluid-filled bottle is so when the pedal is released, fluid is sucked back into the system from either the in-car reservoir or up from the bottle. Once the bubbles are completely gone from the wheel line, tighten the bleeder valve and move onto the next one.
Once you think you’ve got everything just right, lower the car to the ground and give it a test run in a safe location. Check the pedal play, as there should only be 5 to 7mm of play before the pushrod begins to activate the master cylinder. If it is not within acceptable play, adjust it by loosening the clamp bolt and moving the pedal’s stop bracket.
Since brake bleeding procedures and specifications can vary for different vehicles, stop by www.ChiltonDIY.com and bring your maintenance questions to a reassuring rest.
Not only is Ryan Lee Price a freelance writer specializing in automotive journalism and a former long-time magazine editor, he is part of the technical editorial team that provides content for most all of the ChiltonPro and ChiltonDIY products. He currently resides in Corona, California, with his wife Kara and their two children.