chap72 pps Automotive technology at University of Cambridge

start Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 OBJECTIVES: After studying Chapter 72, the reader should be able to: • • • Prepare for ASE Brakes (A5) certification test content area “A” (Hydraulic System Diagnosis and Repair) Explain how to bench bleed a master cylinder Describe the proper brake bleeding sequence Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 OBJECTIVES: After studying Chapter 72, the reader should be able to: • • • Describe the single stroke manual brake bleeding procedure Discuss how to gravity bleed the hydraulic brake system List the steps needed to perform a pressure bleed procedure Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 KEY TERMS: bleeder valve • brake bleeding gravity bleeding power bleeding • pressure bleeding reverse fluid injection single stroke bleeding method • surge bleeding vacuum bleeding Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 BRAKE BLEEDING Brake bleeding is removing any trapped air from the hydraulic  system. Air can get into the hydraulic system whenever any  hydraulic brake line or unit is opened. A common source of air in  the brake system of this type can occur through very small holes in  rubber flexible brake lines Another source is absorption of moisture by brake fluid. When  moisture is absorbed, boiling point of the brake fluid is reduced.  During severe braking, the heat generated can cause the brake  fluid to boil and create air bubbles in the hydraulic brake system Air eventually travels to the highest part of the brake system, if not  restricted by pressure control valves. Air in the system results in a  spongy brake pedal Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 BLEEDING THE MASTER CYLINDER Whenever the master cylinder is replaced or the hydraulicsystem  has been left opened for several hours, the air may have to be bled  from the master cylinder. Bleed the master cylinder “on the  bench” before installing it on the vehicle Figure 72–1 Always clamp a master cylinder in a vise by the mounting flange to prevent distortion of the cylinder bore Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 If bleeding the master cylinder after working on the hydraulic  system, follow these steps: Step #1  Fill the master cylinder with clean brake fluid from a  sealed container up to the recommended “full” level Step #2  Have an assistant slowly depress the brake pedal as you  “crack open” the master cylinder bleed screw starting with the  section closest to the bulkhead. It is very important that the  primary section of the master cylinder be bled before attempting  to bleed the air out of the secondary section of the master cylinder.  Before the brake pedal reaches the floor, close the bleeder valve Step #3  Repeat several times until a solid flow of brake fluid is  observed leaving the bleeder valve. If the master cylinder has no  bleeder valves, outlet tube nuts can be loosened instead Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 NOTE: A proper manual bleeding of the hydraulic system requires that accurate communications occur between the person depressing the brake pedal and the person opening and closing the bleeder valve(s). The bleeder valve (also called a bleed valve) should be open only when the brake pedal is being depressed. The valve must be closed when the brake pedal is released to prevent air from being drawn into the system Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 BRAKE BLEEDER VALVE LOOSENING METHODS Attempting to loosen a bleeder valve often results in breaking  (shearing off) the bleeder valve. Several procedures can be tried  that help prevent the possibility of breaking a bleeder valve.  Bleeder valves are tapered and become wedged in the caliper on  the wheel cylinder housing.  Figure 72–2 Typical bleeder valve from a disc brake caliper The arrows point to the taper section that does the actual sealing It is this taper that requires a shock to loosen If the bleeder is simply turned with a wrench, the bleeder usually breaks off because the tapered part at the bottom remains adhered to the caliper or wheel cylinder Once loosened, brake fluid flows around the taper and out through the hole in the side of the bleeder valve Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 72–3 Typical bleeder locations Note that the combination valve and master cylinder shown not have bleeder valves; therefore, bleeding is accomplished by loosening the brake line at the outlet parts (Courtesy of Allied Signal Automotive Aftermarket) Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Some full­size Ford vehicles have a metering valve with a stem  that must be pushed in to bleed the front brakes, but Ford does not offer a special tool for this purpose. An assistant is needed to override the valve when the front brakes are being bled To install the override tool used on older Chrysler and Ford  vehicles, slip one fork of the tool under the rubber boot, and the  other fork under the valve stem head.  The spring tension of the tool holds the valve open, but allows the  valve stem to move slightly when the system is pressurized. If the  valve is held rigidly open, internal damage will result.   See Figure 72–12 Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 72–12 Pull-out-type metering valves being held out using a special override tool Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 PRESSURE BLEEDING PROCEDURE Just as in manual bleeding, it is important to follow the proper  sequence when pressure bleeding a brake system. Some  manufacturers recommend one sequence for manual bleeding and  another for pressure bleeding.  To pressure bleed a brake system, follow these steps: Consult the equipment manufacturer’s instructions and fill  the pressure bleeder with the proper type of brake fluid Make sure the bleeder is properly sealed and the fluid supply  valve is closed, then use compressed air to pressurize the  bleeder until approximately 30 psi (207 kPa) is indicated on the bleeder gauge Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 If the vehicle is equipped with a metering valve, override it  with the appropriate tool Clean the top of the master cylinder, then remove the master  cylinder cover and clean around the gasket surface. Be  careful not to allow any dirt to fall into the reservoir Fill the reservoir about half full with new brake fluid, install  the proper pressure bleeder adapter on the master cylinder Connect the pressure bleeder fluid supply hose to the adapter,  making sure the hose fitting is securely engaged Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Open the fluid supply valve on the pressure bleeder to allow  pressurized brake fluid to enter the system. Check carefully for fluid leaks that can damage the vehicle finish Slip the plastic hose over the bleeder screw of the first wheel  cylinder or caliper to be bled, and submerge the end of the tube  in the jar of brake fluid Open bleeder screw approximately one­half turn, let fluid run  until air bubbles no longer emerge. Close the bleeder screw 10 Transfer the plastic hose to the bleeder screw of the next wheel  cylinder or caliper in the bleeding sequence, and repeat steps 8  and 9. Continue around the vehicle until the brakes at all four  wheels have been bled Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 11 Remove the metering valve override tool 12 Close the fluid supply valve on the pressure bleeder 13 Wrap the end of the fluid supply hose in a shop towel, and  disconnect it from the master cylinder adapter. Do not spill  any brake fluid on the vehicle finish 14 Remove the master cylinder adapter, adjust the fluid level to  the full point, and install the fluid reservoir cover Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 SURGE BLEEDING Surge bleeding is a supplemental bleeding method used to help  remove air bubbles that resist other bleeding processes. In surge  bleeding, the brake pedal is pumped rapidly to create turbulence  in the hydraulic system Surge bleeding is not recommended for systems filled with  silicone DOT 5 brake fluids. These fluids tend to trap tiny air  bubbles that are very difficult to bleed from the hydraulic system.  Added agitation of surge bleeding only makes the problem worse Surge bleeding requires an assistant to pump the brake pedal, a  bleeder screw wrench, two feet of clear, plastic hose with an inside  diameter small enough to fit snugly over the bleeder screw, and a jar partially filled with clean brake fluid.  Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 To surge bleed a brake system, follow these steps:      Slip the plastic hose over the bleeder screw of the wheel  cylinder or caliper to be bled and submerge the end of the  tube in the jar of brake fluid Open the bleeder screw approximately one­half turn With the bleeder screw open, have your assistant rapidly  pump the brake pedal several times. Air bubbles should come out with the brake fluid While your assistant holds the brake pedal to the floor, close  the bleeder screw Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Repeat steps 2 through 4 at each bleeder screw in the recom  mended order Re­bleed the system using one of the four other methods  previously described Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 What is Reverse Fluid Injection? Reverse fluid injection is a procedure that uses air-or hand-operated injection gun that pushes brake fluid from the bleeder value into the hydraulic system By forcing brake fluid into the bleeder valve, any trapped air is forced upward into the master cylinder This procedure should only be done after a thorough hydraulic system flush Many experts warn that debris and sediment in the hydraulic system can be back-flushed into the ABS hydraulic unit and/or master cylinder Many brake and ABS failures have been caused by forcing old brake fluid back into the system Figure 72–13 A reverse fluid injection unit that is used to bleed hydraulic brake and clutch systems by forcing brake fluid up from the bleeder valve into the master cylinder, thereby forcing any trapped air up and out of the system Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 FLUID CHANGING In addition to removing air, brake systems need to be bled in order  to clean out old and/or contaminated fluid. This process is called  fluid changing or flushing.  Brake fluid changing or flushing is done by bleeding the system  until all of the old fluid is purged from the system. Because fluid  changing requires the ability to flush out contamination and move  a great deal of fluid, the best method to use is pressure bleeding Gravity bleeding is not recommended for fluid changing because  it is slow, and without significant pressure behind the fluid flow,  there is no guarantee that contamination will be completely  flushed from the system Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 To change a vehicle’s brake fluid, remove all the old brake fluid from  the master cylinder reservoir.  Fill the reservoir with new fluid, then  follow the procedures outlined earlier in the chapter for the chosen  method of bleeding Continue to bleed at each wheel until the fluid that emerges from the  bleeder screw is free of any discoloration and contamination Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 SUMMARY Bleeding the brakes means to pump the hydraulic system free  of air A new or replacement master cylinder should be bled before  installing it in the vehicle Bleeder valves are located on all disc brake calipers and drum  brake wheel cylinders The most commonly used method of brake bleeding is the  single stroke bleeding method Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 SUMMARY (cont.) Vacuum bleeding is used to draw the old fluid and any trapped  air from the hydraulic system through the bleeder value Gravity bleeding is an excellent but slow method Pressure bleeding requires adapters and special equipment,  plus the metering valve must be held open to allow fluid to  flow to the front brakes Surge bleeding is the least desirable method of bleeding, but is  necessary at times Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 end Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 ... attempting to loosen the bleeder valve? ?at? ?the same time The punch creates a shock motion that often loosens the taper and  threads? ?of? ?the bleeder valve from the caliper or wheel cylinder.  It is often helpful to first attempt to turn the bleeder valve in the ... ensure that the reservoir remains? ?at? ?least half full Attach a length? ?of? ?plastic tubing to each bleeder screw, place the ends? ?of? ?the tubes in jars to catch drainage Open each bleeder screw approximately one full turn and ... method. In this process, the bleeder screws? ?at? ?all four wheels are  opened? ?at? ?the same time, and the system is allowed to drain  naturally until the fluid coming out? ?of? ?the bleeders is free? ?of? ?air Gravity bleeding is a slow process that can take an hour or more.