Brakes Program 1. Textbook

Transcription

1 Brakes Program 1 Textbook

2 IMPORTANT NOTICE This material provides general directions for collision damage repair using tested, effective procedures. Following them will help assure the reliability of the repair. I-CAR cannot accept responsibility for any individual repair, nor can it warrant to the quality of such repair. Anyone who departs from the instructions in this program must first establish that neither personal safety nor the integrity of the repair of the vehicle is compromised by the choice of methods, tools, or supplies. I-CAR does not endorse or recommend any brands or makes of vehicles, repair equipment and supplies or other products. The appearance of various makes and brand names in any I-CAR material is purely coincidental and is based on the availability of those products at the time of production. All recommendations presented in this program are based upon research programs or upon tests conducted by laboratories, manufacturers, or selected collision repair facilities. If performed as outlined, these recommendations will provide the basis for a thorough, professional repair by the Inter-Industry Conference On Auto Collision Repair (I-CAR) All Rights Reserved 2

3 CONTENTS Module 1 Brake System Parts And Bleeding Procedures... 4 Topic A. Brake System Overview Of Operation... 4 Topic B. Master Cylinders... 4 Topic C. Braking Assists... 6 Topic D. Combination Valves... 7 Topic E. Brake Lines And Hoses... 9 Topic F. Brake Fluid And Bleeding...10 Module 2 Disc Brakes And Drum Brakes Topic A. Disc Brakes...14 Topic B. Drum Brakes...19 Module 3 Parking Brake Parts And Troubleshooting Topic A. Parking Brakes

4 MODULE 1 BRAKE SYSTEM PARTS AND BLEEDING PROCEDURES Topic A. Brake System Overview Of Operation Topic B. Master Cylinders Lining Caliper Brake Lines Master Cylinder Mounting Hardware Drum Assembly MASTER CYLINDERS Refer to screen B-1v of your Student CD-ROM for a video on master cylinder operation. Parking Brake INOPERATIVE BRAKE SYSTEM Always tag a vehicle that has an inoperative brake system to prevent vehicle operation in the shop. Rotor Combination Valve Parking Brake Cable Wheel Cylinder A-2 A brake system uses a variety of parts to transfer hydraulic fluid from the master cylinder to each wheel. A vehicle in motion has kinetic energy. A brake system applies friction at each wheel to slow down or stop vehicle motion. The friction turns kinetic energy into heat which is absorbed and dissipated in the brake system parts. Service brake systems use hydraulic pressure to slow or stop wheel movement. Fluid is used because it does not compress. Therefore, when pressure is exerted on fluid, it is transmitted equally in all directions in the system, regardless of distance. There are several parts that make up a master cylinder. There is a reservoir, a pushrod, and primary and secondary pistons. The reservoir supplies the master cylinder with fluid. Some systems have a remote reservoir that uses a low-pressure supply line to supply fluid to the master cylinder. The pushrod connects the brake pedal and power booster. Lastly, the primary and secondary pistons force pressure into the brake lines. As the piston is moved by the brake pedal pushrod, the brake fluid forward of the piston seals is forced into the brake lines, raising the pressure in the lines. Vehicles are built with multi-circuit braking systems. These systems have front and rear, or diagonally separated brake systems, to ensure partial braking is retained in case of a brake line failure. Pressure is created in the brake system by applying pressure to the brake pedal. This creates hydraulic pressure in the master cylinder by forcing the piston against the fluid. The master cylinder creates hydraulic pressure in the brake lines. The brake lines carry hydraulic pressure to the wheel cylinders and calipers which change hydraulic pressure into motion. The piston in the caliper or wheel cylinder moves outward as a result of the hydraulic pressure. This stops the vehicle by forcing the friction material against the rotor or drum. 4

5 Another example is brake fluid contamination. This often results from a petroleum-based fluid being added to the system, causing internal rubber parts to swell. Both of these examples may cause a rising brake pedal and brake drag. A low brake pedal or a sinking pedal can be caused by a master cylinder with an internal leak (from either collision damage or corrosion). This can also be caused by an external leak, incorrectly adjusted brake shoes, or a faulty parking brake mechanism on four-wheel disc systems. B-2 Collision damaged master cylinders are typically replaced if damaged. Collision damage to the master cylinder may include broken lines, detached reservoirs, bent housings, dirt contamination, and cowl panel damage. A sinking brake pedal will slowly drop to the floor when depressed. This problem is typically caused by an internal leak in the master cylinder or a leak in one of the brake lines. The master cylinder assembly may be rebuilt or overhauled depending on the type. If the master cylinder must be replaced, it is usually replaced as a unit. To replace the master cylinder assembly: 1. Disconnect the brake lines and tightly seal the ends. 2. Unbolt the master cylinder assembly. 3. Carefully remove the assembly from the vehicle. 4. Fill and bleed the new master cylinder on the bench. B-3 A faulty master cylinder may be indicated by a rising, sinking, or low brake pedal. BENCH BLEEDING A MASTER CYLINDER Refer to screen B-7v of your Student CD- ROM for a video on how to bench bleed a master cylinder. A faulty master cylinder can be the cause of some brake problems. One example is a blocked master cylinder fluid port. This may prevent fluid from returning to the master cylinder. TROUBLESHOOTING: A SINKING BRAKE PEDAL Refer to screen B-8 of your Student CD-ROM for an interactive exercise on troubleshooting a sinking brake pedal. 5

6 Topic C. Braking Assists When repairing collision damaged power-assist systems, damaged boosters must be replaced. During installation, be sure to properly adjust the pushrod, or improper braking may result. Refer to the vehicle service manual for proper adjustment procedures. TROUBLESHOOTING: A HARD PEDAL Refer to screen C-4 of your Student CD-ROM for an interactive exercise on how to troubleshoot a hard pedal. C-1 The braking assist unit shown is attached to the master cylinder. A hard pedal can be described as excessive pressure required by the driver to stop a vehicle. Hard pedal can be caused by collision damage to power brake boosters. Braking-assist systems are designed to reduce the pedal effort required to apply the brakes. Typically, engine vacuum is used to help move the master cylinder piston. When vacuum is not available, hydraulic pressure may be used. Typically, the assist unit is installed between the cowl and master cylinder. Vacuum-assisted power brakes have a vacuum booster that contains a diaphragm. When operated, vacuum is applied to one side of the diaphragm and atmospheric pressure to the other. Vacuum is taken from the engine intake manifold. If additional vacuum is required, a separate vacuum pump may be necessary to supply adequate vacuum for some engines. Collision damage to power brake boosters include vacuum leaks, visible damage to the unit, and possible internal damage. Symptoms of damage to the power assists include the following: Hard pedal (more than normal effort is required to stop vehicle) Hissing noise Sensitive brakes Slow releasing pedal Pedal chatter Idles bad or high (vacuum leak) C-5 This Mercedes braking system is designed to provides full braking force for decreased stopping distance. Some vehicles have an assisted emergency braking system. This system recognizes an emergency braking situation by measuring the rate of the brake pedal being depressed and comparing it to normal brake pedal movement. The system automatically applies full braking force to help decrease stopping distances. When the driver pushes the brake pedal much quicker than normal, signaling an emergency, the computer applies an electronic valve on the brake booster system for full braking. At the same time, the anti-lock braking system is engaged to prevent wheel lockup. 6

7 Topic D. Combination Valves Combination valves are used in both disc/disc and disc/drum systems. Disc/drum systems use a combination valve along with a pressure differential warning light switch, proportioning valve, and a metering valve. Disc/disc systems use a combination valve with a proportioning valve and pressure differential warning light switch. The combination valve helps ensure correct distribution of braking pressure. If the hydraulic system has lost fluid after a line has broken and been repaired, the pressure switch in the combination valve might not be centered. This will likely cause a dash warning lamp to light, indicating a problem. D-1 A combination valve is used to distribute pressurized hydraulic fluid from the master cylinder to each of the wheels. Vehicles without anti-lock brake systems (ABS) typically have some type of control valve or switch in the brake system called a combination valve. The combination valve is connected in line with the master cylinder and calipers or wheel cylinders. Parts of a combination valve generally include a metering valve, residual pressure valve, proportioning valve, and a pressure differential switch. The pressure from the master cylinder flows into the combination valve and is distributed to each of the wheels. The combination valve may activate a warning light when there is fluid loss from a broken line. BRAKING SYSTEM OVERVIEW Refer to screen D-2v of your Student CD- ROM for a video on how a brake system operates. D-4 Collision damage to the combination valve usually requires the part to be replaced. Collision damage to a combination valve and its parts includes bent and twisted lines, cracked casings, and damaged electrical switches and wiring. Combination valves are not usually repairable and must be replaced if damaged. When removing and tightening brake lines a flarenut wrench should be used to prevent rounding the corners off of the nut. 7

8 D-6 Front-end dive is generally caused by a faulty combination valve. Always use caution when test driving a vehicle that has had brake work done or requires brake work (troubleshooting). Because the vehicle s braking capabilities are unknown it could be extremely easy to exceed them. D-5 A faulty combination valve will send brake pressure to only part of the brake system. A faulty combination valve can cause jerky or uneven braking. This can also be described as a severe reaction to pedal pressure. A faulty combination valve causes uneven braking by sending brake fluid pressure to only one circuit. If pressure is sent to only the front brakes, it causes front-end dive. If pressure is sent to only the rear brakes, it causes rear-end squat and greatly reduced braking effect. The effects a faulty combination valve has on braking will depend on how the hydraulic systems are divided (such as front to rear, diagonally, or a combination of the two). Always use caution when test driving a vehicle that has had brake work done or requires brake work (troubleshooting). Because the vehicle s braking capabilities are unknown it could be extremely easy to exceed them. Front-end dive is when the front end of the vehicle excessively dips down when the brakes are applied. This problem is typically caused by a faulty combination valve. The combination valve must be replaced if damaged. To replace the combination valve: 1. Disconnect and seal all brake lines. 2. Install a new valve. 3. Bleed the system. 4. Follow the vehicle maker s procedures for centering the valve s internal switch. TROUBLESHOOTING: FRONT-END DIVE Refer to screen D-7 of your Student CD-ROM for an interactive exercise on troubleshooting front-end dive. 8

9 Topic E. Brake Lines And Hoses Brake hoses may be torn, split, or overextended from suspension damage during a collision. All damaged hoses must be replaced. Make sure the hose is routed the same as the original factory routing and not twisted during tightening. A vehicle with a firm, but sinking pedal may have a leak in one of the brake lines. Depending on its location, a leak may contaminate the friction material. If the leak is large enough, the brake pedal could go to the floor losing at least 50% of its braking action. E-1 Brake line ends have either an ISO flare fitting (top photo) or a double flare fitting (bottom photo). The brake lines and hoses transmit fluid pressure from the master cylinder to the calipers or wheel cylinders. Brake line ends are either ISO flare fittings or double flare fittings. Double flare fittings are typically found on older model vehicles. ISO FLARE FITTINGS Select the Demonstration Icon found on screen E-1 of your Student CD-ROM for a video of the ISO flare fittings demonstration. Brake lines are made of double-wall steel. When making brake lines, make sure they are carefully formed and routed to avoid damage. Brake lines may be kinked or broken during a collision. If a brake line is damaged it must be replaced. The line must be routed the same as the original factory routing. Do not straighten kinked lines. Brake hoses are designed to be connected from rigid brake lines to moveable parts. They are flexible to move with wheel and suspension movement. E-3 Internal blockage of a brake line cause brake drag. A vehicle that pulls to one side while braking may have a partially blocked brake line which restricts fluid flow. This can be caused by a dent, bend, or internal blockage in a brake line. In many cases, the master cylinder and booster will provide enough pressure to force the brake fluid past the obstruction to apply the brakes. However, the fluid pressure may not be able to release due to the obstruction once the brakes are released. This would result in a constant pressure to the caliper or wheel cylinder and cause a dragging brake. Brake drag is easiest to diagnose and most noticeable in slippery conditions. This condition may also be noticeable on dry pavement as an intermittent pull immediately after the brakes are released. 9

10 When inspecting brake lines, look for leaks, dents, kinks, corrosion, cracks, or wear. Dents or kinks in a line will restrict fluid flow. Also, tighten loose fittings and supports. If damage exists, replace the line. Replace brake lines and hoses with those that are identical in size and material. It is also important that the routing follows the original factory routing and that the correct mounting clips are used. Brake lines may be available pre-made. Brake hose damage is not always apparent and can be difficult to diagnose. Internal damage can create a valve inside the line with a loose piece of rubber. When this happens a flap of inner liner allows fluid to flow in one direction, but not return. This condition makes the vehicle feel like a brake is dragging. Brake hoses can also break down structurally and swell as the brakes are applied. This condition would cause a soft pedal. TROUBLESHOOTING: VEHICLE PULL Refer to screen E-5 of your Student CD-ROM for an interactive exercise on diagnosing vehicle pull. Topic F. Brake Fluid And Bleeding E-4 During visual inspection of brake hoses, remember that hoses may be damaged internally without showing signs of external damage. Brake hoses should be carefully inspected and replaced if damaged. Look for leaks, kinks, cracks, bulging, or wear. When replacing brake hoses: Brake fluid is an integral part of any brake system. When replacing brake fluid, use new brake fluid only and make sure it the proper type (for example DOT 3, 4, or 5). The fluid must remain uncontaminated and should not be exposed to the atmosphere any more than necessary. Do not shake the container before use, as it causes air bubbles that would be added into the system. Lastly, dispose of used fluid according to local regulations. WATER ABSORPTION IN BRAKE FLUID Select the Demonstration Icon found on screen F-1 of your Student CD-ROM for a video of the water absorption in brake fluid demonstration. 1. Unbolt the hose from the connections. 2. Install a new hose. 3. Replace or reinstall all washers. 4. Route the hose identical to the previous hose. 5. Test wheel and suspension movement for any interference with the new brake hose. 6. Bleed the system. 10

11 DOT 4 brake fluid has a higher boiling point than DOT 3 and can be mixed with DOT 3. It is also designed to absorb more moisture than DOT 3. DOT 5 brake fluid, which is purple in color, is silicone based, does not absorb moisture, and compresses more than DOT 3 or DOT 4. It is not compatible with DOT 3 or DOT 4 and is used and recommended by only a few vehicle makers. Collision damage analysis should also consider brake fluid contamination. Brake fluids (except silicone- DOT 5) absorb moisture when exposed to air. F-2 Check for fill marks when adding brake fluid to a system. When adding brake fluid to a the master cylinder reservoir, leave about 6 mm ( 1 4") space between the fluid and the top of the reservoir. Some plastic reservoirs are clear and have fill marks. Review brake fluid cautions on the container before adding brake fluid. When placing the cover back on the master cylinder, refold the rubber seal (if necessary). Note that the fluid level may go down as brake linings wear. If the fluid level is improper, however, air may enter the system. Brake fluid is considered hazardous waste and must be disposed of according to local regulations. Do not mix discarded brake fluid with any other substance. DOT 3 brake fluid is non-petroleum based and has additives that aid internal system lubrication and prevent corrosion. It is designed to maintain its viscosity in high temperatures and absorb any moisture in the system. Moisture in a braking system can vaporize, creating air in the system and lowering the brake fluid boiling point. This process can cause corrosion on brake system parts. To help prevent moisture contamination, do not store the container after it has been opened. Dispose of any remaining brake fluid. Take care to ensure that brake fluid is not spilled on painted surfaces. If fluid is spilled, flush with water immediately. F-7 There are tools available that allow a technician to test for moisture content in brake fluid. Moisture greatly reduces the boiling point of brake fluid. Absorption of even 4 percent moisture can reduce the boiling point 50 percent. This can happen to a system that has been left open, with the amount of time dependent on the humidity level. A soft brake pedal has a springy or spongy feel when depressed. A soft brake pedal is typically caused by worn or damaged brake hoses that expand under pressure. It may also be caused by damage to the caliper assembly or vapor in the system. When the boiling point of the brake fluid is reached, vapor can be air, water vapor, or brake fluid vapor. With air or vapor in the hydraulic system, braking efficiency is reduced because the air or vapor compresses under pressure. 11