Bleeding brakes

Last Updated on July 22, 2024 by Mutiara

The role of the brake fluid within the braking system is to transfer the force from the master cylinder to the corners of the car. A vital characteristic of brake fluid that allows it to perform the task properly is its ability to maintain a liquid state and resist compression. Keeping the fluid in top condition requires bleeding the brakes, which is the process in which a small valve is opened at the caliper (or wheel cylinder) to allow controlled amounts of brake fluid to escape the system.

Brakes are bled to release air that sometimes becomes trapped within the lines. Air only enters the lines if there is a compromise of the system’s sealing (as when flex lines are removed or replaced), because when fluid boils, it will instead create fluid vapor. Vapor in the brake fluid, like air, will create an efficiency loss in the braking system. For this article, air will be used to describe both air and fluid vapor.

When air (or vapor) becomes present within the lines, it creates inefficiencies within the system because, unlike liquid, air can be compressed. So when enough air fills the lines, input at the pedal merely causes the air to compress instead of creating pressure at the brake corners. In other words, when air is present within the system, the efficiency and effectiveness of the braking system is reduced. Usually, a small amount of air within the brake system will contribute to a “mushy” or “soft” pedal (since less energy is required to compress the air than is required to move fluid throughout the brake lines.) If enough air enters the brake system, it can result in complete brake failure.

Air can enter as a result of a service procedure or an upgrade, such as replacing the stock flex lines with stainless steel braided lines. But often it is the result of high temperatures that causes brake fluid components to boil, thus releasing gasses from the boiling fluid into the brake hydraulic system.

Table of Contents

Brake Fluid Selection

Glowing rotors on a racecar demonstrate the tremendous heat generated by brake system components.

In theory, even simple water would work as brake fluid, since it is a liquid and cannot be compressed. However, it is important to remember that the fundamental function of the braking system is to convert kinetic energy into heat energy through friction. And the reality of this process is that certain parts of the braking system will be exposed to very high temperatures. In fact, it is not uncommon to see rotor temperatures during a race as high as 1200 degrees Fahrenheit, which can raise the temperature of the brake fluid to well over 300 degrees Fahrenheit. Since the boiling point of water is 212 degrees Fahrenheit, it is easy to see that water within the brake system could boil easily, and therefore release gases into the brake pipes, thus reducing the efficiency of the system. Water would also present a big problem from freezing in cold weather.

The “obvious” solution to this problem is to utilize a fluid that is less sensitive to temperature extremes. Hence the development of “brake fluid.” However, there unfortunately is no such thing as a “perfect” brake fluid. And like most things in the world, the addition of certain beneficial characteristics usually brings compromises in other areas. In the case of brake fluid, we generally must balance the fluid’s sensitivity to temperature against its cost and its impact upon other components within the system.

Stated more bluntly, it is possible to reduce a fluid’s sensitivity to temperature by varying the ingredients of the fluid. However, certain combinations of ingredients can significantly increase the cost of the fluid and may react with OEM materials to damage seals and induce corrosion throughout the braking system.

The chemical composition and minimum performance requirements of the fluid are generally indicated through a rating such as DOT3, DOT4, or DOT5. The DOT-rating itself is assigned after a series of government tests. However, this rating is NOT intended to indicate boiling points, even though higher DOT ratings generally do correspond with higher boiling points. Perhaps more importantly, the DOT rating does indicate the base compound of the brake fluid, which allows manufacturers to specify fluid types which are less likely to react negatively to known materials used within a particular braking system.

The chemical components in brake fluid that tend to be less sensitive to temperature extremes also tend to attract and absorb water. So even though the fluid itself is unlikely to boil (most glycol-based DOT3 fluids have a “dry boiling point” around 400 degrees Fahrenheit), the water that it absorbs over time tends to boil easily (at 212 degrees Fahrenheit). It is this characteristic of absorbing moisture that leads to the measure known as the wet boiling point. The wet boiling point is the equilibrium boiling point of the fluid after it has absorbed moisture under specified conditions. Because brake fluid will absorb moisture through the brake system’s hoses and reservoir, evaluation of the wet boiling point is employed to test the performance of used brake fluid and the degradation in it’s performance. This is the reason why brake fluid on racecars still requires to be bled frequently, even though racing brake fluid can cost up to $ 75 a bottle. Just because the fluid is expensive, does not mean it has greater service intervals over regular fluid.

As one might guess, “racing” fluids will use relatively “aggressive” chemical compositions which will tend to have higher wet boiling points and higher costs, while the average street fluids will use more conservative compositions which will have lower wet boiling points and lower costs. In some cases, such as a purpose built racecar, the tradeoffs of using the expensive racing fluid is outweighed by the competitive advantages. But for the average driver – whose driving style is less likely to induce brake temps as high as those seen on the track, the costs of the fluids and potential wear-and-tear factors upon system components may justify the use of a more conservative fluid with a lower wet boiling point.

When to bleed the brakes

Under normal operating conditions, and without brake system modifications, typical OEM braking systems have been designed to NOT require bleeding for the life of the vehicle unless the system is opened for repair or replacement. If the vehicle is driving around town or the highway to work, chances are there is no need to bleed. There are a few European vehicles which do recommend replacement on a semi-regular basis for other reasons though, so be sure to check in your owner’s manual or at your service center for your particular application.
Older vehicles, and even some fairly new ones can develop brake fluid contamination from normal use. This can be seen by inspecting the brake reservoir. If the fluid is dark and not very clear, or any color such as green or brown, it should be flushed entirely until the fluid coming out of the bleeder screws is pure and clear.
Those who choose to autocross or drive in a sporting manner may choose to upgrade their brake fluid and bleed on an annual basis, this is a good start of the season maintenance item for low-speed competitors.
If the vehicle sees significant amounts of high-speed braking, or if you choose to participate in driver schools and/or lapping sessions, bleeding prior to each event is a sound decision.
Dedicated race cars should be bled after every track session.

Bleeding procedure

Requirements An assistant to push the brake pedal. Box-end wrench suitable for the vehicle’s bleeder screws. An offset head design usually works best. Extra brake fluid. 1 pint for bleeding, 3 pints for complete replacement. 12-inch long section of clear plastic tubing that can fit tightly over the bleeder screws. Disposable bottle for waste fluid. One can of brake cleaner. One assistant (to pump the brake pedal).

Unless the master cylinder is being replaced this procedure is the same whether the vehicle has ABS or not.

Vehicle Preparation and Support

Loosen the lug nuts of the road wheels and place the entire vehicle on jackstands. Be sure that the car is firmly supported before going ANY further with this procedure!
Remove all road wheels.
- Removal of the wheels is not necessary on all vehicles, as some will have the clearance to perform the following steps with the wheels still on the vehicle. However in most cases this is recommended.
Install one lug nut backward at each corner and tighten the nut against the rotor surface. Note that this step is to limit caliper flex that may distort pedal feel.
Open the hood and check the level of the brake fluid reservoir.
Add fluid as necessary to ensure that the level is at the MAX marking of the reservoir. Do not let the reservoir become empty at any time during the bleeding process!

Bleeding Process

A hose attached to the bleeder screw on a typical caliper.

Begin at the corner furthest from the driver and proceed in order toward the driver. While the actual sequence is not critical to the bleed performance it is easy to remember the sequence as the farthest to the closest. This will also allow the system to be bled in such a way as to minimize the amount of potential cross-contamination between the new and old fluid.
1. Right rear.
2. Left rear.
3. Right front.
4. Left front.
Locate the bleeder screw at the rear of the caliper body (or drum brake wheel cylinder). Remove the rubber cap from the bleeder screw, and don’t lose it.
Place the box-end wrench over the bleeder screw. An offset wrench works best, since it allows the most room for movement. Make sure the wrench is on tight and the proper size, as bleeder screws can sometimes be hard to work loose from corrosion. Stripping bleeder screws is very easy and often results form using the wrong wrench.
Place one end of the clear plastic hose over the nipple of the bleeder screw.
Place the other end of the hose into the disposable bottle.
Place the bottle for waste fluid on top of the caliper body or drum assembly. Hold the bottle with one hand and grasp the wrench with the other hand.
Instruct the assistant to apply. The assistant should pump the brake pedal three times, hold the pedal down firmly, and respond with “applied.” Instruct the assistant not to release the brakes until told to do so.
Loosen the bleeder screw with a brief 1/4 turn to release fluid into the waste line. The screw only needs to be open for one second or less. (The brake pedal will “fall” to the floor as the bleeder screw is opened. Instruct the assistant in advance not to release the brakes until instructed to do so.)
Close the bleeder screw by tightening it gently. Note that one does not need to pull on the wrench with ridiculous force. Usually just a quick tug will do.
Instruct the assistant to release the brakes. Do NOT release the brake pedal while the bleeder screw is open, as this will suck air back into the system!
The assistant should respond with “released.”
Inspect the fluid within the waste line for air bubbles.
Repeat the bleeding process until air bubbles are no longer present. Be sure to check the brake fluid level in the reservoir after bleeding each wheel! Add fluid as necessary to keep the level at the MAX marking. A standard bleed usually involves repeating the process 5-10 times per wheel.
When all four corners have been bled, spray the bleeder screw or any other parts that came in contact with the brake fluid with brake cleaner and wipe dry with a clean rag. Leaving the area clean and dry will make it easier to spot leaks through visual inspection later. Try to avoid spraying the brake cleaner DIRECTLY on any parts made of rubber or plastic, as the cleaner can make these parts brittle after repeated exposure.
Reinstall the rubber caps on all bleeder screws.
Test the brake pedal for a firm feel. Bleeding the brakes will not necessarily cure a “soft” or “mushy” pedal since pad taper and compliance elsewhere within the system can contribute to a soft pedal. But the pedal should not be any worse than it was prior to the bleeding procedure.
Be sure to inspect the bleeder screws and other fittings for signs of leakage. Correct as necessary.
Properly dispose of the used brake fluid.
Re-install all four road wheels.
Raise the entire vehicle and remove jackstands. Torque the lug nuts to the manufacturer’s recommended limit. Re-install any hubcaps or wheel covers.

With the vehicle on level ground and with the car NOT running, apply and release the brake pedal several times until all clearances are taken up in the system. During this time, the brake pedal feel may improve slightly, but the brake pedal should be at least as firm as it was prior to the bleeding process.
Road test the vehicle to confirm proper function of the brakes.

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