OBJECTIVESAfter studying Chapter 6, the reader should be able to:1. Prepare for the Brakes (A5) ASE certification test.2. List the types of brake fluids.3. Describe where armored brake line is used.4. Discuss the differences between double flare and ISO flare.5. Explain how flexible brake lines should be handled during service.6. List the precautions necessary when handling or disposing of brake fluid.7. Discuss the types of rubber that are used in brake system components.
BRAKE FLUIDBrake fluid boiling point is one of the most critical aspects and ratings for brake fluid. Brake fluid is hygroscopic, meaning that it readily absorbs water. As brake fluid ages, it absorbsmoisture, which lowers its boiling point and causes increased corrosion of the brake system components.
BRAKE FLUID TYPESBrake fluid is a polyalkylene–glycol–ether mixture called polyglycol for short. All polyglycol brake fluid is clear to amber in color. Brake fluid has to have the following characteristics:A high boiling pointA low freezing pointNo ability to damage rubber parts in the brake system
BRAKE FLUID SPECIFICATIONSAll automotive brake fluid must meet federal Motor Vehicle Safety Standard 116. The Society of Automotive Engineers (SAE) and the Department of Transportation (DOT) have established brake fluid specification standards.
DOT 3DOT 3 brake fluid is the type most often used.1. DOT 3 absorbs moisture. According to SAE, DOT 3 can absorb 2% of its volume in water per year. Moisture is absorbed by the brake fluid through microscopic seams in the brake system and around seals. Over time, the water will corrode the system and thicken the brake fluid. The moisture can also cause a spongy or no brake pedal if is heated to the point of boiling.
2. DOT 3 must be used from a sealed (capped) container. If allowed to remain open for any length of time, DOT 3 will absorb moisture from the surrounding air. If the can has been open for a length of time, use a new can of fluid.CAUTION: DOT 3 brake fluid is a very strong solvent and can remove paint! Care is required when working with DOT 3 brake fluid to avoid contact with the vehicle’s painted surfaces. It also takes the color out of leather shoes.
DOT 4This brake fluid is formulated for use by all vehicles, imported or domestic. DOT 4 is polyglycol based but has borate esters added to provide an extra buffer for the fluid against acids that can form in the moisture that has been absorbed in the fluid when it is heated. DOT 4 can often be used where DOT 3 is used and even though the two types of brake fluid are compatible and miscible (able to be mixed), some vehicle manufacturers recommend that DOT 3 and DOT 4 not be mixed.
DOT 5.1DOT 5.1 is a nonsilicone-based polyglycol fluid and is clear to amber in color. This severe duty fluid has a boiling point of over 500ºF, equal to the boiling point of silicone-based DOT 5 fluid. Unlike DOT 5, DOT 5.1 can be mixed with either DOT 3 or DOT 4, according to brake fluid manufacturer’s recommendations.CAUTION: Some vehicle manufacturers do not recommend the use of or the mixing of other types of polyglycol brake fluid and specify the use of DOT 3 brake fluid only. Always follow the vehicle manufacturer’s recommendation.
DOT 5DOT 5 brake fluid is commonly called silicone brake fluid and is made from polydimethylsiloxanes. It does not absorb any water, and is therefore called nonhygroscopic.
DOT 5 brake fluid is purple (violet) in color to distinguish it from DOT 3 or DOT 4 brake fluid. Silicones have about three times the amount of dissolved air as glycol fluids (about 15 % of dissolved air versus only about 5 % for standard glycol brake fluid).
1. Silicone brake fluid has an affinity for air; therefore, it is more difficult to bleed the hydraulic system of trapped air.2. The trapped air expands with increasing temperature. This causes the brake pedal to feel “mushy” because the pressure exerted on the hydraulic system simply compresses the air in the system and does not transfer the force to the wheel cylinders and calipers as it should.3. The air trapped in the silicone brake fluid can also “off-gas” at high altitudes, causing a mushy brake pedal and reduced braking performance.4. DOT 5 brake fluid should not be mixed with any other type of brake fluid.5. DOT 5 does not affect rubber parts and will not cause corrosion.6. DOT 5 is expensive.
Old brake fluid (four years old) often has a boiling point under 300°F (150°C). With the air temperature near 100°F (38°C), it does not take much more heat to start boiling the brake fluid.
BRAKE FLUID INSPECTION AND TESTINGThe brake fluid should be inspected regularly, including the following items.1. Proper Level.2. Color/Condition.
BRAKE FLUID SERVICE PROCEDURES AND PRECAUTIONS1. Store brake fluid only in its original container.2. Before opening a brake fluid container, remove any dirt, moisture, or other contamination from the top and outside of the container.3. When a brake fluid container is empty, it should be discarded4. Do not transfer brake fluid to any other container that may have contained oil, kerosene, gasoline, antifreeze, water, cleaners, or any other liquids or chemicals.5. Do not reuse brake fluid that has been siphoned from another vehicle or drawn out during a brake bleeding operation.6. Use only fresh new brake fluid for flushing the hydraulic brake system.
BRAKE FLUID HANDLING AND DISPOSALBrake fluid spilled on the floor should be cleaned up using absorbent material and the material disposed of in the regular trash. Brake fluid becomes a hazardous waste if spilled onto open ground, where it can seep into groundwater.
RUBBER TYPESVehicles use a wide variety of rubber in the braking system, suspension system, steering system, and engine. Rubber products are called elastomers. Some are oil- and grease-resistant elastomers and can be harmed by brake fluid, while others are brake-fluid resistant and can swell or expand if they come in contact with oil or grease.
BRAKE LINESHigh-pressure double-walled steel brake lines or high-strength flexible lines are used to connect the master cylinder to each wheel. The steel brake lines are also called brake pipes or brake tubing.
All double-walled brake tubing is plated with tin, zinc, or other similar substances for protection against rust and corrosion.CAUTION:Copper tubing should never be used for brake lines. Copper tends to burst at a lower pressure than steel.
When replacing steel brake line, new steel tubing should be used and a double lap flare or an ISO flare completed at each end using a special flaring tool.
Brake line can also be purchased in selected lengths already correctly flared. They are available in different diameters, the most commonly used being 3/16 in. (4.8 mm), 1/4 in. (6.4 mm), and 5/16 in. (7.9 mm) outside diameter (O.D.).
Brake line diameter is also very important and replacement lines should be the same as the original. Many vehicle manufacturers use larger-diameter brake lines for the rear brakes because the larger line decreases brake response time. Response time is the amount of time between the pressure increase at the master cylinder and the pressure increase at the brakes.
The purpose of the coils is to allow movement between the brake components without stress that could lead to metal fatigue and brake line breakage.
FLEXIBLE BRAKE HOSEFlexible brake hoses are used on each front wheel to allow for steering and suspension movement and at the rear to allow for rear suspension travel.
Flexible brake hose is made from synthetic yarn (poly vinyl alcohol, abbreviated PVA) that is braided into position from multi-end yarn spindles. By braiding the yarn, all of the strands operate in tension and, therefore, have great strength to withstand braking system pressure over 1,000 psi (6,900 kPa).
A typical brake hose has an inner tube for conveying the brake fluid and a cushion liner that is between the braided layers to prevent the braids from chafing. An outside jacket is made from rubber and protects the reinforcement fabric from moisture and abrasion.The outside covering is also ribbed as part of the manufacturing process to hide surface blemishes.
A constricted brake hose can cause the brakes to remain applied, thereby causing excessive brake pad wear and unequal braking. A constricted flexible brake line can also cause the vehicle to pull to one side. The hoses should also be inspected for external wear and damage.
CAUTION: Never allow a disc brake caliper to hang by the flexible brake line. Damage to the line can result. Always use a wire to support the weight of the caliper.
SUMMARY1. Most brake fluid is amber in color and polyglycol based.2. DOT 3 is the type of brake fluid most often recommended for use.3. Brake fluid absorbs moisture over time and can remove paint.4. DOT 4 and DOT 5.1 are also polyglycol-based fluids, whereas DOT 5 is silicone based.5. Brake fluid should be checked for proper level and color and tested for condition or for contamination.6. Brake fluid should be disposed of according to local and state guidelines.7. Most brake systems use EPR- or SBR-type rubber that is compatible with brake fluid but they can swell in size if exposed to mineral oil such as power steering fluid, engine oil, or automatic transmission fluid.8. Brake lines are double-walled steel with a rust prevention coating with either a double lap flare or an ISO flare end.9. Flexible brake hose is constructed of braided synthetic yarn and EPDM rubber.
REVIEW QUESTIONS1. What is the difference between DOT 3, DOT 4, DOT 5.1, and DOT 5 brake fluid?2. List four things that should be done during a thorough inspection of the brake fluid.3. List four precautions that should be followed when handling brake fluid.4. What are the two types of flares used on brake lines?5. Why are some brake lines coiled?