What is "bleeding the brakes" and why is that sometimes necessary?

Bleeding the brakes is flushing the old brake fluid out of the master cylinder, brake lines, calipers and wheel cylinders and replacing it with fresh fluid.

Bleeding is necessary for two reasons:

1. 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. On some ABS-equipped vehicles, special bleeding procedures may be required (which also requires special equipment in some cases such as a scan tool to cycle the ABS solenoids).

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. Most professionals use power bleeding equipment because it’s faster and easier.

2. To remove moisture contamination. Brake fluid needs to be replaced periodically because DOT 3 and 4 brake fluids are glycol-based and absorb moisture over time. This occurs whether a vehicle is driven 30,000 miles 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 (a good reason not to do so unnecessarily).

After only a year of service, DOT 3 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 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. Once brake fluid turns to 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.

The danger of fluid boil is greatest in front-wheel drive cars because of the higher operating temperatures that are generated in today’s downsized front brakes, and because the hydraulic system is split diagonally. 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.

DOT 3 brake fluid, which has long been used in most domestic cars and light trucks, has a minimum dry boiling point of 401 degrees F. A 3% level of water contamination will lower this by 25% or 100 degrees! DOT 4 "extra heavy-duty" brake fluid, which is used in many European cars, has a higher dry boiling point of 446 degrees F. DOT 4 soaks up moisture at a slower rate than DOT 3 but suffers a greater drop in heat resistance as moisture builds up. Only 2% moisture in DOT 4 fluid will lower its boiling point by almost 50% or 200 degrees!

CAUTION: Use the type of brake fluid specified by the vehicle manufacturer. Never substitute DOT 3 for DOT 4. But you can safely substitute DOT 4 for DOT 3.

Though the owner’s manuals for most domestic vehicles have no specific time or mileage recommendations for replacing brake fluid, recommending a change every two years for preventative maintenance is a good way to minimize the danger of fluid boil and internal corrosion in the brake system. At the very least, the fluid should always be replaced when the brakes are relined.

Some people say using DOT 5 silicone fluid eliminates moisture contamination problems. The premium-priced fluid, which is silicone based, does not absorb moisture and is theoretically a"lifetime" brake fluid. What’s more, DOT 5 fluid has a higher dry boiling point of at least 500 degrees F. and a wet boiling point of 356 degrees F. But DOT 5 silicone brake fluid is very expensive (up to ten times as much as regular brake fluid!). It does not mix with DOT 3 or 4 fluid, which means all the old fluid has to be removed if switching to DOT 5 to prevent "slugs" of moisture-contaminated DOT 3 and 4 fluid from forming in the system. And it is not recommended for any vehicle equipped with ABS because it contains a higher percentage of dissolved air that may cause foaming when the fluid is cycled rapidly. Other than that, it’s great stuff.