"Bleeding" a brake system refers to driving all of the air out of the system. Modern automobile braking systems consist of a master cylinder in the engine compartment which pushes brake fluid (a hydraulic fluid mixture consisting primarily of various glycols) through a series of tubes (brake lines) which then actuate the brake cylinders. This is true whether you have disc brakes, drum brakes, or both.
Air has two characteristics which make it undesirable in a brake system. The first is that it is a gas (actually, a mixture of gases) and unlike liquids, it is compressible. The brake system depends on your pedal force being transmitted through the lines via hydraulic pressure, and the compression of the air will reduce this effect. The second is that air as it is found in the atmosphere contains water vapor - Brake fluid is hygroscopic and absorbs water. Water lowers the boiling point of the brake fluid, and when your brake fluid boils, its life expectancy comes to a close. For this reason, leaving the cap off of the brake fluid reservoir (found on the master cylinder) will reduce the lifetime of your brake fluid.
Air can get into the system in two ways, either through failure of a brake system component or through error during maintenance. If you perform the brake bleeding procedure incorrectly you will introduce air into the system through the top and/or the bottom. If any of the many plastic or rubber components (flexible lines or "flex-lines", seals, and often portions of the master cylinder) are heated too much too often or exposed to sunlight they will dry and crack, and fluid will be exchanged for air at that point.
Please keep in mind that your brakes are a critical system. If you bleed them incorrectly you can impair their function and thus your stopping distance, which can lead to injury to yourself and/or others, as well as property damage. If you do not feel confident in your ability to perform this procedure, please take your car to a shop and have it done by professionals. I accept no liability for an incorrectly (or for that matter correctly) performed brake job.
This guide is written specifically for passenger cars and light trucks with four wheels, but the same principles apply to any vehicle which uses a hydraulic braking system, including (but not limited to) motorcycles and assorted off-road vehicles such as ATVs. The difference lies largely in the number of wheels one must visit. Motorcycles, for example, often have a linkage which actuates the rear brake, and a hydraulic system to control the front, and naturally they have only one front wheel. The process applies equally to vehicles with and without anti-lock brakes (ABS), though some such vehicles may have additional procedures to follow. When in doubt, consult a service manual, preferrably the factory service manual (FSM).
You will need:
- A partner to help you with the bleeding process. There are any number of fun devices which are intended to make brake bleeding easier, including pumps to push the fluid up into the master from the wheel. They tend to not work very well. One person will need to operate the brake pedal while the other opens and closes the bleeder valves.
- A jack, to raise up the car
- A jack stand or blocks, to support the car.
- The proper size wrench for your bleeder valves. A small crescent wrench can solve this need quite adequately. Most bleeder valves have a 3/8" head on them. If your bleeder valves are frozen shut (usually with rust and/or dirt mixed with brake fluid) you will need either a six-sided box end wrench or a brake line wrench (like a box end wrench, but with a small opening to allow you to get it past hard metal brake lines), which grips more surfaces of the hex portion of the bleeder.
- Brake Fluid - Use the appropriate rating of fluid for your car, which is usually DOT 3. For performance applications, you will want to use a higher-rated fluid, as it has a higher boiling point. Make sure that if you are going to a performance brake fluid that it is compatible with whatever brake fluid was last in the system, otherwise you will need to flush the system which is difficult. The easiest way to avoid this problem is to avoid synthetic brake fluid (unless you have an exotic car which came with it) though there are certainly synthetic fluids which are compatible with ordinary glycol-based brake fluid. If your brake fluid is incompatible with the old stuff, the old stuff will "pill" in your brake system, meaning form small solid chunks, which can clog lines and render your brake system ineffective.
- A piece of clear flexible tubing which will fit on your bleeder valve nipples. Typically, 3/8" OD clear vinyl tubing will do the job.
- A container for used brake fluid. Never reuse brake fluid, as it is hygroscopic and water lowers its boiling point. The stuff is toxic and will need to be disposed of properly - Your local landfill or transfer station can do this for you. Your trash company may also have special pickup days for toxic chemicals. I like translucent plastic milk jugs for storage of toxic chemicals, especially those I do not like, because they are easily identifiable through the container, and you can write on them with a sharpie.
Brake bleeding is simple. There is however a right way to do it. Having bled and re-bled the brakes on a number of vehicles, including a 1963 Chevrolet T-10 truck, and a 1989 Nissan 240SX Fastback, I am in a great position to explain the process.
First, several important words on brake fluid; The stuff is nasty. You probably will get some on your hands; Wash it off at the earliest opportunity. More importantly, DO NOT GET IT ON YOUR PAINT. It WILL cause your paint to come off.
Next, you should find out the suggested bleeding order. Most vehicles suggest a specific order in which you are intended to bleed the brakes to ensure that air is efficiently evacuated from the system. In the absence of this information, you should bleed the back brakes first, and you should start on the driver's side; So for a USDM vehicle, you should bleed the rear left first, followed by the rear right, then the front left, and finally the front right.
While an actual rebuild is very different, whether you are bleeding disc or drum brakes, the process is identical. Jack up the wheel you're working on (you can of course jack up the entire vehicle at once) and support it with a jack stand or with some sort of blocks, and then remove the wheel.
You will now be looking either at the disc brake caliper and disc, or at a brake drum. Either way, what you want to find now is the bleeder valve. This should look like a long-headed hex bolt with a nipple on top of it. You only need to turn this valve a quarter to a half a turn in order to open it.
Attach your clear line to the nipple on the bleeder valve, and run it into the container. Now we begin the actual bleeding process.
The person in the car should now press and release the brake pedal a couple of times. Now, with the pedal in the up position, you should open the bleeder valve a quarter to a half turn with your 3/8" wrench. Now the person in the car should push in the brake pedal and hold it down; Now you close the valve. If the pumper raises the pedal before you close the valve, some air will be drawn back into the system (or, if you're lucky, some of the fluid in your line) and possibly compromise your bleeding job. The tighter your clear line fits onto your bleeder fitting, the less likely this is to happen. The pumper can pump the brakes a couple times, and then with them up, you open it again. Repeat as necessary until the fluid comes out completely free of bubbles. The purpose of pumping the pedal with the bleeder closed is to unify the air bubbles in the system to aid in pumping them out. It sounds superstitious, but it makes a significant difference.
Repeat the process at all four wheels, making sure to keep the brake master cylinder topped off. If it is ever emptied, you must start over bleeding at least that wheel, if not the entire system. This depends on how things are laid out. If you have a single master cylinder, then you will need to start all over again. If it's a dual master, then you probably only need to do the front or back over again, depending on where you are. This is definitely not always true, and it depends on how dry you have run it, how many fluid reservoirs your system has, and one or two other factors. If in any doubt whatsoever, re-bleed the entire system.
When you finish, the pedal feel should be stiff, demonstrating that there is no (relatively) easily-compressed air in the lines. Reassemble all that you have disassembled, and do a test drive at low speed to make sure that your braking performance is all that it should be. During this time, if your braking performance begins to deteriorate at all, go bleed the system again. If you bleed the system three times and it does not stop happening, you have a bad brake system component, and you need to inspect the entire system, cylinders, calipers lines and all, for leaks or damage.
As a final note, far and away the best and easiest way to bleed your brakes is to install Russell Speed Bleeder valves, or a similar product. These are replacement bleeder valves which contain a ball bearing check valve, which only allows fluid to exit the system when open. They take a 3/8" wrench, and are made out of high-quality brass. The best thing about them is that they reduce brake bleeding to a one-person process - Everything is done as normal, except you don't need to close the valve before letting up the pedal, because of the check valve, meaning you don't have to open and close the bleeder screw. It should cost you about US$20 to $50 to replace all four on basically any vehicle. You can of course purchase a small check valve and attach it to the bleeder valve with a small piece of tubing, but you will then need to put hose clamps on the ends of the interim tubing because otherwise you will not get a good seal.
Bicycles
A brief note on bleeding bicycles: It is fairly common now for off-road bicycles to have hydraulically operated disc brakes. Each manufacturer has their own service procedure with wildly different tools to perform the service. On Shimano brakes, the top cap is removed from the master cylinder and a special clamp/seal arrangement with the reservoir attached is actually clamped onto the master (where the brake lever is attached.) On Hayes brakes, fluid is added by attaching a squeeze bottle full of fluid to the caliper, and attaching a dirty-fluid bottle to the master cylinder with a small aluminum adapter plug. In general however, the bicycle should be mounted on a stand, and rotated so that the caliper bleeder valve points towards the sky; the master cylinder's clamps are loosened and it is rotated so that its top tap points up as well.