By Tom Benford
Editor's Note: When working on any of your car's vital safety systems, if you're not sure what you're doing, have someone with experience work with you. There's no margin for error.
Brake fluid can be thought of as a moveable rod which functions to activate the mechanical parts that actually stop the vehicle. Bleeding the brakes flushes the old brake fluid out of the master cylinder, brake lines, calipers and wheel cylinders and replaces it with fresh fluid.
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.
This is an overview at a glance of the brake system components and the procedures used to bleed the system. Be sure to read the entire article however!
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). However, only very late model collector vehicles (e.g., 1984 and later Corvettes) are equipped with ABS braking systems.
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.
The Mityvac is a trigger-operated vacuum pump that lets one person successfully bleed brakes at the wheel, without depressing the pedal or wasting fluid. This versatile little pump moves about 1 cu. in. of fluid with each stroke, pulling out dirt, old fluid, and air. It develops and holds approximately 25 inches of vacuum. The kit includes the pump, brake bleeding adapter package user's manual, 3 lengths of 1/4" I.D. tubing (1 1/2", 3", and 18"), 3" of 5/32" tubing and a reservoir jar with transfer and storage lids.
And secondly, 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 50,000 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 passed seals and through microscopic pores in hoses. It also enters every time the fluid reservoir is opened (which is a real good reason not to open it unnecessarily).
To give you an idea of what a problem moisture in the brake fluid can be, consider this: 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!
The Motive Products power bleeder is and effective and easy-to-use pressurized brake bleeder. Just remove the master cylinder lid, attach adapter, fill the reservoir, pressurize reservoir with attached pump, and open the bleed fittings. The bleeder is available for older domestic, modern domestic (shown) and foreign car models. It comes with instructions, a 1-year manufacturer's warranty and it is made in the USA — something rare these days.
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.
DOT 3 brake fluid, which has long been used in most domestic cars and light trucks, has a minimum dry boiling point of 401° Fahrenheit. A 3% level of water contamination will lower this by 25% or 100°. DOT 4 "extra heavy-duty" brake fluid, which is used in many European cars, has a higher dry boiling point of 446° 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°.
CAUTION: ALWAYS Use the type of brake fluid specified by the vehicle manufacturer. Never substitute DOT 3 for DOT 4, although 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, changing it 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.
Speed Bleeders are automotive bleeder screws with built-in check valves that make bleeding your brakes truly a one person job. Simply remove your OEM bleeder screw and replace it with Speed Bleeder. When you are ready to bleed your brakes, loosen the Speed Bleeder 1/4 turn. Pump the brake lever or pedal until bubble free fluid is seen. Close the Speed Bleeder and you're done.
Some proponents claim that using DOT 5 silicone fluid totally eliminates moisture contamination problems. The premium-priced fluid, which is silicone based, does not absorb moisture and is thus theoretically a "lifetime" brake fluid. DOT 5 fluid also has a higher dry boiling point of at least 500° F. and a wet boiling point of 356° F. On the downside, however, 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 must be removed if switching to DOT 5 to prevent "slugs" of moisture-contaminated DOT 3 and 4 fluid from forming in the system. And DOT 5 fluid 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. Aside from that, it is excellent brake fluid.
OK, so now that you know a bit more about brake fluid, here's how to bleed the brakes on your collector vehicle.
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