Bleeding Brake Systems

Brake System Knowledge

This guide is intended to provide you with a basic understanding of the various methods of how to bleed brakes; not to provide in-depth knowledge of brake systems. It will assume that you are familiar with the basic components of a modern brake system such as the master cylinders, brake calipers, wheel cylinders, rotors, drums, pads, shoes, et cetera. It is often helpful--and sometimes necessary--to lift the vehicle and remove the wheels in order to gain access to the bleeder screws. Use extreme caution to ensure your car or truck is properly supported at all times.

Brake System Problems

Over time, the hydraulic brake fluid in your car's brake system will absorb moisture. Moisture lowers the boiling point of brake fluid and allows the brake system's steel components to corrode. If the boiling point of the brake fluid in your vehicle drops too low, the heat produced when you apply the brakes will cause the brake fluid to boil. When this happens, your pedal may start to feel spongy, require greater force to stop the vehicle, or may fail completely! Furthermore, if the corrosion manages to eat through a steel component such as a brake line, you will lose all brake pressure and will be unable to stop the vehicle! To keep your brakes in tip-top shape, it is recommended that you have your brake fluid flushed as often as once every two years.

Brake Bleeding vs. Flushing

The term "bleeding" means to remove the air locked in the brake system; whereas, the term "flushing" means to remove all of the old fluid and replacing it with new, clean fluid. Mechanics will usually bleed the brakes after a component has been replaced and the hydraulic system had to be "opened". To flush your vehicle's brake system, simply continue bleeding each wheel until the fluid runs clean. Considering the high cost of replacing brake system components, it is often wise to do a complete flush.

Brake Bleeding Methods

There are three methods of bleeding that can be done without prohibitively expensive equipment: manual brake bleeding, vacuum brake bleeding, and gravity brake bleeding. Whether bleeding a master cylinder on a bench, or a brake caliper or wheel cylinder on the car or truck, the principle is the same. You want to force air and fluid out and add new fluid, all while preventing fresh air from entering the system. Regardless of the method you choose, you'll quickly realize the trick is to keep the brake fluid moving in only one direction; from the master cylinder through to the brake calipers or wheel cylinders. Be sure to keep topping up the master cylinder with brake fluid as you bleed each wheel and after you are finished to prevent it from running low and pulling fresh air into the system. When the system is full of clean brake fluid and there is no air trapped inside, the brake pedal should be high and firm.

Method 1: Manual Bleeding

Manual brake bleeding is the most common method of bleeding brakes; however, you will need to enlist the help of an assistant. With your assistant sitting in the driver's seat, repeat the following six steps a number of times on each brake until you are sure there is no air trapped in the system. Use a narrow block of wood behind the pedal to prevent it from travelling all the way to the floor. Lastly, place a three foot piece of vinyl hose on the end of the bleeder screw to direct old fluid into a plastic container.

1. Instruct your assistant to pump the brake pedal for thirty seconds
2. Instruct your assistant to press and hold the brake pedal firmly
3. Open a bleeder screw and let the air and old fluid escape
4. Close the bleeder screw
5. Instruct your assistant to release the brake pedal
6. Wait fifteen seconds

Method 2: Vacuum Bleeding
Vacuum bleeding has the advantage of being a one-person job, but requires a special tool known as a vacuum tester or brake bleeding kit. The tool is fairly inexpensive and can be ordered online by clicking on the link above. Whereas manual brake bleeding requires you to "push" brake fluid out; the object of vacuum bleeding is to "pull" brake fluid out. Repeat the following three steps on each brake.

1. Connect the hose from the vacuum tool to the bleeder screw and then open the bleeder screw
2. Pump the tool until the fluid leaving the bleeder screw runs clean and is free of bubbles
3. Close the bleeder screw and disconnect the vacuum tool hose

Method 3: Gravity Bleeding

Gravity bleeding is the easiest method of bleeding brakes. Simply, repeat the following two steps on each brake. Unfortunately, leaving the screw open for any length of time will allow it to absorb moisture. Additionally, air bubbles may be trapped in the system and need to be worked out using a vacuum or pressure bleeding method. The gravity bleeding method can be used when only a brake caliper or wheel cylinder was replaced.

1. Open one bleeder screw at a time and wait until the air works its way out of the system
2. Close the bleeder screw when clean brake fluid runs from the bleeder screw

Master Cylinders

In most circumstances, you will not need to bleed a master cylinder. This is usually only done when the master cylinder has been replaced or left open for several hours. New master cylinders should be bench bled before being installed on the vehicle. Regardless of whether you are bench bleeding the cylinder or bleeding it in the vehicle, you will need to make or purchase a set of short brake lines which screw into the master cylinder and loop back up into the reservoir. Use the outlet fittings in place of bleeder screws to bleed the master cylinder. These fittings should be opened while pressing the piston or brake pedal and closed when the piston is allowed to return. If you are bleeding the master cylinder on a bench, use a round rod inside the master cylinder to press the piston. When finished, install the new master cylinder and bleed each wheel completely.

Bleeder Screws

Disc brakes are bled through bleeder screws located in the brake calipers. If the bleeder screw is old it may be difficult to remove or loosen without breaking, as the taper of the bleeder screw will seize against its seat in the caliper. Sometimes tapping the caliper near the bleeder screw (but not on the bleeder screw) with a hammer, while trying to turn the screw, will jolt or break the taper. It is not recommended to use heat to remove the screw as you can damage the seals in the caliper. Drum brakes are bled through the bleeder screw on the back of the wheel cylinder. Like brake calipers, the bleeder screw may be seized in place and difficult to remove. If a bleeder screw breaks it may be possible to drill it out and retap the threads. Otherwise, a new caliper or wheel cylinder will need to be installed.

Brake Fluid

You should find three common types of brake fluid at your local auto parts store: DOT3, DOT4, and DOT5. DOT3 is the most common and should also be the cheapest. Unfortunately, DOT3 brake fluid also absorbs moisture the fastest and has the lowest boiling point. DOT4 is also fairly common but is a little more expensive. DOT4 is designed to absorb moisture slower and has a higher boiling point. DOT5 is not as common and is the most expensive. DOT5 does not absorb any moisture and has the highest boiling point. DOT3 and DOT4 are clear to amber in color, while DOT5 is purple. The personal at your local auto parts store should be able to determine which type of fluid is required for your vehicle and how much will be required. You can use DOT4 wherever DOT3 is specified, and DOT5 only on cars or trucks not equipped with ABS (unless otherwise indicated in your owner's manual). If you do choose to switch to a higher quality brake fluid, it is strongly recommended that you perform a complete brake system flush. Never use a mineral based oil, such as engine oil or transmission fluid, in place of proper brake fluid. Mineral oil will cause all of the rubber parts in the hydraulic system to swell up and leak. Lastly, take care when handling brake fluid. It can damage the finish of a painted surface.

Disclaimer
While we make every attempt to insure this information is complete and accurate, it is impossible to account for every scenario. Please consult with a local technician before attempting to perform any work you are not qualified to do. Automobiles can be hazardous to work on; be sure to take all necessary safety precautions. Failure to do so may result in property damage or personal injury.