chap69 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 69, the reader should be able to: • • • Prepare for ASE Brakes (A5) certification test content area “A” (Hydraulic System Diagnosis and Repair) State Pascal’s Law Describe the function, purpose, and operation of the master cylinder 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 69, the reader should be able to: • • • Explain how hydraulic force can be used to supply high pressures to each individual wheel brake Describe the process of troubleshooting master cylinders and related brake hydraulic components Explain how a quick take-up master cylinder works 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: bore master cylinder • breather port • bypass port • bypassing compensating port diagonal split master cylinder • dual split master cylinder fast-fill master cylinder • filler port hydraulic system • master cylinder Pascal’s Law • pedal free play • pedal height • pedal reserve distance • piston assemblies 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 KEY TERMS: quick take-up master cylinder replenishing port self-apply • step-bore master cylinder vent port 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 HYDRAULIC PRINCIPLES In addition to mechanical advantage provided by leverage, all vehicles use hydraulic pressure to increase brake application force All braking systems require that a driver’s force is transmitted to the drum or rotor attached to each wheel The force exerted on the brake pedal varies due to strength and size of the driver Figure 69–1 Hydraulic brake lines transfer the brake effort to each brake assembly attached to all four wheels Engineers design brake systems to require less than 150 lb of force (68 kg) from the driver, yet provide the force necessary to stop a heavy vehicle from high speed 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 NONCOMPRESSIBILITY OF LIQUIDS Hydraulic systems use liquids to transmit motion For all practical purposes, a liquid cannot be compressed No matter how much pressure or force is placed on a quantity of liquid, its volume will remain the same Figure 69–2 Because liquids cannot be compressed, they are able to transmit motion in a closed system This fact enables liquids in a closed system to transmit motion 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 Liquids cannot be compressed, but any air trapped in the system can be compressed The hydraulic system is air-contaminated Even though piston A is moved in., piston B will not move if the load on it is greater than the pressure of the air in the system Figure 69–3 Hydraulic systems must be free of air to operate properly If the load on piston B is 50 pounds per square inch (psi), the movement of piston A must compress the air in the system to that same pressure before piston B will move A brake system must be air free or there will be serious problems 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 PASCAL’S LAW The hydraulic principles that permit a brake system to function were discovered by a French physicist, Blaise Pascal (1632–1662) Pascal’s Law states that “when force is applied to a liquid confined in a container or an enclosure, the pressure is transmitted equal and undiminished in every direction.” Assume a force of 10 lb is exerted on a piston with a surface area of square inch (sq in.) Since this force measured in lb or Newtons (N) is applied to a piston with an area measured in square inches (sq in.), the pressure is the force divided by the area or “10 pounds per square inch” (psi) It is this “pressure” that is transmitted, without loss, throughout the entire hydraulic system See Figure 69– 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 69–4 A one-pound force exerted on a small piston in a sealed system transfers the pressure to each square inch throughout the system In this example, the 1-lb force is able to lift a 100-lb weight because it is supported by a piston that is 100 times larger in area than the small piston 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 Spongy Brake Pedal A spongy pedal with a larger than normal travel indicates air in the lines Check for leaks and bleed the air from the system as discussed later in this chapter Lower Than Normal Brake Pedal A brake pedal that travels downward more than normal and then gets firm is an indication that one circuit of the dual-circuit hydraulic system is probably not working Check for leaks in the system and repair as necessary Another possible reason is an out-of-adjustment drum brake allowing too much pedal travel before the shoes touch the brake drum NOTE: A lower than normal brake pedal may also be an indication of air in the hydraulic 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 Sinking Brake Pedal If the brake pedal sinks all the way to the floor, suspect a defective master cylinder that is leaking internally This internal leakage is often called bypassing because the brake fluid is leaking past the sealing cup NOTE: A sinking brake pedal, on a vehicle equipped with an antilock braking system (ABS), could be caused by a defective dump valve 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 DISASSEMLY OF THE MASTER CYLINDER Many master cylinders can be disassembled, cleaned, and restored to service NOTE: Check the vehicle manufacturer’s recommendation before attempting to overhaul or service a master cylinder Many manufacturers recommend replacing the master cylinder as an assembly Step #1 Remove the master cylinder from the vehicle, being careful to avoid dripping or spilling brake fluid onto painted surfaces of the vehicle Dispose of all old brake fluid and clean the outside of the master cylinder 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 Step #2 Remove the reservoir, if possible, as shown here Figure 69–37 Using a pry bar to remove the reservoir from the master cylinder (Courtesy of Allied Signal Automotive Aftermarket) Step #3 Remove the retaining bolt that holds the secondary piston assembly in the 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 Step #4 Depress the primary piston with a blunt tool such as a Phillips screwdriver, a rounded wooden dowel, or an engine pushrod Use of a straight blade screwdriver or other nonrounded tool can damage and distort the aluminum piston CAUTION: If holding the master cylinder in a vise, use the flange area Never clamp the body of the master cylinder Step #5 Remove the snap ring and slowly release the pressure on the depressing tool Spring pressure should push the primary piston out 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 Step #6 Remove the master cylinder from the vise and tap the open end of the bore against the top of a workbench to force the secondary piston out of the bore If necessary, use compressed air in the outlet to force the piston out CAUTION: Use extreme care when using compressed air The piston can be shot out of the master cylinder with a great force Figure 69–38 Whenever disassembling a master cylinder, note the exact order of parts as they are removed Master cylinder overhaul kits (when available) often include entire piston assemblies rather than the individual seals 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 INSPECTION AND REASSEMBLY OF THE MASTER CYLINDER Thoroughly clean the master cylinder and any other parts to be reused (except rubber components) in clean denatured alcohol If the bore is OK, replacement piston assemblies can be installed into the master cylinder after dipping them into clean brake fluid NOTE: While most master cylinder overhaul kits include the entire piston assemblies, some kits just contain the sealing cups and/or O-rings Always follow the installation instructions that accompany the kit and always use the installation tool that is included to prevent damage to the replacement seals 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 Step #1 Install the secondary (smaller) piston assembly into the bore, spring end first Figure 69–39 Piston assembly (Courtesy of Allied Signal Automotive Aftermarket) NOTE: While most master cylinder overhaul kits include the entire piston assemblies, some kits just contain the sealing cups and/or O-rings Always follow instructions that accompany the kit and always use the installation tool included to prevent damage to the replacement seals 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 69–40 To reinstall the reservoir onto a master cylinder, place the reservoir on a clean flat surface and push the housing down onto the reservoir after coating the rubber seals with brake fluid (Courtesy of Allied Signal Automotive Aftermarket) Step #2 Install the primary piston assembly, spring end first Step #3 Depress the primary piston and install the snap ring Step #4 Install the stop bolt Step #5 Reinstall the plastic reservoir, if equipped, as shown at left Continued Automotive Technology: Principles, Diagnosis, and Service, 3rd Edition By James D Halderman Step #6 Bench bleed the master cylinder This step is very important See Figure 69–41 ©©2008 2009Pearson PearsonEducation, Education,Inc Inc Pearson PearsonPrentice PrenticeHall Hall- -Upper UpperSaddle SaddleRiver, River,NJ NJ07458 07458 Figure 69–41 Bleeding a master cylinder before installing it on the vehicle The master cylinder is clamped into a bench vise while using the rounded end of a breaker bar to push on the pushrod end with bleeder tubes down into the brake fluid Master cylinders should be clamped on the mounting flange as shown to prevent distorting the master cylinder bore 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 Check for Bypassing If a master cylinder is leaking internally, brake fluid can be pumped from the rear chamber into the front chamber of the master cylinder This internal leakage is called bypassing When fluid bypasses, the front chamber can overflow while emptying the rear chamber Whenever checking the level of brake fluid, not think that a low rear reservoir is always due to an external leak Also, a master cylinder that is bypassing (leaking internally) will usually cause a lower than normal brake pedal NOTE: Brake fluid can drip from the outlet of the master cylinder and could drip onto the vehicle Brake fluid is very corrosive and can remove paint Use fender covers and avoid letting brake fluid touch any component of the vehicle 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 INSTALLING THE MASTER CYLINDER After the master cylinder has been bench bled, it can be installed in the vehicle Tighten the fasteners to factory specifications Figure 69–42 Installing a master cylinder Always tighten the retaining fastener and brake lines to factory specifications Bleed the system as needed 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 During a typical brake application, only about teaspoon (5 ml or cc) of brake fluid actually is moved from the master cylinder and into the hydraulic system Pascal’s Law states that: “When a force is applied to a liquid confined in a container or enclosure, the pressure is transmitted equally and undiminished in every direction.” Master cylinder reservoirs are large enough for the brakes to be worn completely down and still have a small reserve The front port of the master cylinder is called the compensating port and the rear port is called the inlet port 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.) Brake system diagnosis should always start with checking for venting (compensation) Dual split master cylinders that separate the front brakes from the rear brakes are used on rear-wheel-drive vehicles Diagonal split master cylinders that separate right front and left rear from the left front and right rear brakes are used on frontwheel-drive vehicles Some master cylinders can be rebuilt, but the cylinder bore should not be honed unless recommended by the manufacturer 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