Brake fluid within braking system is to activate the mechanical parts that actually stop the vehicle. In technical terms, the fluid transfers the force from the master cylinder to corners of the car which allows the fluid to perform its task properly and the ability to maintain liquid state and resist compression. In order to keep the fluid in top condition, many enthusiasts have been taught to “bleed their brakes” but many have never stopped to ask the question “why?
Why Bleed the Brakes?
The term “bleeding the brakes” refers to 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.
Bleeding is necessary for two reasons
First, to remove air bubbles that may have entered the system while repairs were being made, because of a leak or because the fluid level got too low. The air must be removed because it is compressible and can prevent a full, firm pedal.
The individual brake lines must be bled in a specified sequence (which varies from vehicle to vehicle depending on the design of the brake system) to remove all the air from the lines. The brakes can be bled manually by attaching a piece of clear tubing to the bleeder screw on each caliper and wheel cylinder, opening the screw and manually stroking the brake pedal to force fluid through the lines, or with power bleeding equipment.
And secondly, to remove moisture contamination. Brake fluid needs to be replaced periodically because brake fluids are glycol-based and absorb moisture over time. This occurs whether a vehicle is driven 50,000 Km a year or just sits in a garage because fluid contamination is a function of time and humidity rather than mileage. Moisture enters the system past seals and through microscopic pores in hoses. It also enters every time the fluid reservoir is opened.
To give you an idea of what a problem moisture in the brake fluid can be, consider this: after only a year of service, brake fluid may contain as much as 2% water. After 18 months, the level of contamination can be as high as 3%. And after several years of service, it’s not unusual to find brake fluid that has soaked up as much as 7% to 8% water. Many vehicles that are six, seven or eight years old have never had the brake fluid changed!
As the brake fluid soaks up moisture, it thickens and becomes less able to withstand heat and corrosion. The result is a significant drop in the fluid’s boiling temperature, which may, under the right conditions allow the fluid to boil in the calipers or wheel cylinders. Once brake fluid boils and turns into vapor, the bubbles cause an increase in the distance the pedal must travel to apply the brakes. This condition should not be confused with “brake fade” that occurs when the brake linings get too hot as a result of prolonged braking. Brake fade requires greater and greater pedal effort to stop the vehicle while fluid boil increases pedal travel and makes the pedal feel soft or mushy. Semi-metallic linings compound the heat problem by conducting heat from the rotors to the calipers. If the fluid contains a lot of moisture and can’t take the heat, it’ll probably boil.
Owner’s manuals for most vehicles does not specify time or Km recommendations for changing brake fluid, changing after every two years for maintenance is a good way to minimize danger of fluid boil and internal corrosion in the brake system. The brake should always be replaced when the brakes are relined.
So, now that you understand the need behind bleeding your brakes, let us present just one procedure that can be utilized when servicing your own car. Note that unless you are replacing your master cylinder, the procedure is the same whether you have a vehicle equipped with ABS or not…
You will need the following tools:
- Clear plastic tubing
- Box wrench
- Pint of brake fluid
- Turkey baster
- Clear plastic bottle
- Spacer (e.g., a piece of 1×4 or 2×4 lumber)
- Another person to be your helper
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.
- 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!
- Begin at the corner furthest from the driver and proceed in order toward the driver. (Left rear, Right rear, Left front, Right front.) 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.
- 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.
- 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 ¼ 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. Note: 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.
- Continue the bleeding process (steps 11 through 16) 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. (Typically, one repeats this process 5-10 times per wheel when doing a ‘standard’ bleed.)
- Move systematically toward the driver – right rear, left rear, right front, left front – repeating the bleeding process at each corner. Be sure to keep a watchful eye on the brake fluid reservior! Keep it full!
- When all four corners have been bled, spray the bleeder screw (and any other parts that were moistened with spilled or dripped 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.
- 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 waste fluid as you would dispose of used motor oil. Important: used brake fluid should NEVER be poured back into the master cylinder reservoir!
Vehicle Wrap-Up and Road Test
- 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. Use caution the first time you drive your car after modification to ensure the proper function of all vehicle systems!