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  • Figure 105-1 Typical vacuum brake booster assembly. The vacuum hose attaches to the intake manifold of the engine. The brake pedal travel sensor is an input sensor for the antilock braking system.
  • Figure 105-2 A wide brake pedal allows two-foot braking if power assist is lost.
  • Figure 105-3 Atmospheric pressure varies with altitude.
  • Figure 105-4 A belt-driven auxiliary vacuum pump.
  • Figure 105-5 An electrically powered vacuum pump.
  • Figure 105-6 Vacuum brake boosters operate on the principle of pressure differential.
  • Figure 105-7 The charcoal filter traps gasoline vapors that are present in the intake manifold and prevents them from getting into the vacuum chamber of the booster.
  • Figure 105-8 (a) Many vacuum brake booster check valves are located where the vacuum hose from the engine (vacuum source) attaches to the vacuum booster.
  • Figure 105-8 (b) This one-way valve prevents the loss of vacuum when the engine is off. The diaphragm inside allows air to flow in one direction only.
  • Figure 105-10 Cross-sectional view of a typical vacuum brake booster assembly.
  • Figure 105-11 In the release position (brake pedal up), the vacuum is directed to both sides of the diaphragm.
  • Figure 105-12 Simplified diagram of a vacuum brake booster in the apply position. Notice that the atmospheric valve is open and air pressure is being applied to the diaphragm.
  • Figure 105-13 Cross section of a vacuum brake booster in the hold position with both vacuum and atmospheric valves closed. Note that the reaction force from the brake fluid pressure is transferred back to the driver as a reaction force to the brake pedal.
  • Figure 105-14 Cutaway showing a dual-diaphragm (tandem) vacuum brake booster.
  • Figure 105-16 When the brake assist function operates, the brake force is much higher than normal.
  • Figure 105-17 Typical adjustable pushrod. This adjustment is critical for the proper operation of the braking system. If the pushrod is too long, the brakes may be partially applied during driving. If the rod is too short, the brake pedal may have to be depressed farther down before the brakes start to work.
  • Figure 105-16 When the brake assist function operates, the brake force is much higher than normal.
  • Figure 105-17 Typical adjustable pushrod. This adjustment is critical for the proper operation of the braking system. If the pushrod is too long, the brakes may be partially applied during driving. If the rod is too short, the brake pedal may have to be depressed farther down before the brakes start to work.
  • Figure 105-18 (a) Typical vacuum brake booster pushrod gauging tool. (a) The tool is first placed against the mounting flange of the master cylinder and the depth of the piston determined.
  • Figure 105-18 (b) Typical vacuum brake booster pushrod gauging tool. (b) The gauge is then turned upside down and used to gauge the pushrod length. Some vacuum brake boosters do not use adjustable pushrods. If found to be the incorrect length, a replacement pushrod of the correct length should be installed.
  • Figure 105-19 A holding fixture and a long tool being used to rotate the two halves of a typical vacuum brake booster.
  • Figure 105-20 Exploded view of a typical dual-diaphragm vacuum brake booster assembly.
  • Figure 105-21 Hydro-Boost unit attaches between the bulkhead and the master cylinder and is powered by the power steering pump.
  • Figure 105-22 Exploded view of the Hydro-Boost unit.
  • Figure 105-23 A Hydro-Boost hydraulic booster in the unapplied position.
  • Figure 105-24 A Hydro-Boost hydraulic booster as the brakes are applied.
  • Figure 105-25 A Hydro-Boost hydraulic booster in the holding position.
  • Figure 105-26 A typical Hydro-Boost hydraulic line arrangement showing the pump, steering gear, and brake booster assembly.
  • Figure 105-27 Pressure and flow analyzer installation to check the power steering pump output.

Halderman ch105 lecture Halderman ch105 lecture Presentation Transcript

  • POWER BRAKE UNIT OPERATION, DIAGNOSIS, AND SERVICE 105
  • Objectives
    • The student should be able to:
      • Prepare for the Brakes (A5) ASE certification test content area “D” (Power Assist Units Diagnosis and Repair).
      • List the parts of a vacuum brake booster.
      • Describe how a vacuum brake booster operates.
  • Objectives
    • The student should be able to:
      • Explain how to test a vacuum brake booster.
      • Describe how a hydraulic brake booster operates.
  • THE NEED FOR POWER BRAKE ASSIST
  • The Need for Power Brake Assist
    • Most vehicles with disc brakes are power assisted
    • The most commonly used power-assisted units are vacuum operated
    • With a power brake booster, the brake pedal ratio is decreased and the master cylinder bore size is increased to reduce pedal effort, while greatly increasing pedal reserve
  • The Need for Power Brake Assist
    • Power boosters do not alter the hydraulic system and they still allow braking even if the booster fails or its power supply is cut off
    • Some vehicles with power brakes have a brake pedal that is wide enough to allow two-foot braking should the booster fail
  • Figure 105-1 Typical vacuum brake booster assembly. The vacuum hose attaches to the intake manifold of the engine. The brake pedal travel sensor is an input sensor for the antilock braking system.
  • Figure 105-2 A wide brake pedal allows two-foot braking if power assist is lost.
  • PRINCIPLES OF VACUUM
  • Principles of Vacuum
    • Most vacuum-powered brake boosters get their vacuum supply from the engine intake manifold, which lowers the air pressure within the cylinder, and the higher pressure air outside the engine flows in through the intake manifold in an attempt to fill the low-pressure area
  • Principles of Vacuum
    • The difference in pressure between two areas is called a pressure differential
    • Gasoline-powered internal-combustion engines normally operate with a low-pressure area, or partial vacuum, in the intake manifold
  • Principles of Vacuum
    • The term vacuum is used to refer to any pressure lower than atmospheric pressure, which is approximately 14.7 pounds per square inch (PSI) at sea level
  • Figure 105-3 Atmospheric pressure varies with altitude.
  • Principles of Vacuum
    • Measuring Vacuum
      • Vacuum is measured in inches of mercury (in. Hg) or in millimeters of mercury (mm Hg)
  • Principles of Vacuum
    • Measuring Vacuum
      • Vacuum is a measurement of the pressure differential between the lower pressure inside the tube, and the higher pressure outside
  • Principles of Vacuum
    • Measuring Vacuum
      • A perfect vacuum is about 30 in. Hg (762 mm Hg) and is never achieved in an engine’s intake manifold
  • Principles of Vacuum
    • Measuring Vacuum
      • Manifold vacuum varies with throttle position
      • The lowest manifold vacuum (highest pressure) occurs when the throttle is wide open with the engine under load
  • Principles of Vacuum
    • Measuring Vacuum
      • The highest manifold vacuum (lowest pressure) may be as much as 24 in. Hg (610 mm Hg) when the vehicle is rolling rapidly downhill in gear with the throttle closed
  • Principles of Vacuum
    • Measuring Vacuum
      • Manifold vacuum at idle typically falls between 15 and 20 in. Hg (381 and 508 mm Hg), and most vacuum brake boosters are designed to operate with vacuum levels in this range
  • Principles of Vacuum
    • Booster Vacuum Supply
      • Vacuum boosters get their vacuum supply from the engine intake manifold
      • Diesel engines run unthrottled and have little or no intake manifold vacuum
  • Principles of Vacuum
    • Booster Vacuum Supply
      • If a vehicle with a diesel engine is equipped with a vacuum-powered brake booster, it must also be fitted with an auxiliary vacuum pump
  • Principles of Vacuum
    • Booster Vacuum Supply
      • Some small gasoline-powered and diesel engines use a belt-driven add-on pump
      • Some vehicles use an electric vacuum pump, which is turned on and off by a pressure switch on the booster, so they operate only when needed
  • Figure 105-4 A belt-driven auxiliary vacuum pump.
  • Figure 105-5 An electrically powered vacuum pump.
  • VACUUM BOOSTER THEORY
  • Vacuum Booster Theory
    • Vacuum boosters use the principle of pressure differential to increase brake application force
    • The typical vacuum booster has a power chamber separated into two smaller chambers by a flexible diaphragm
  • Vacuum Booster Theory
    • When air pressure is greater on one side of the diaphragm than the other, a pressure differential is created
    • In an attempt to equalize pressure in the two chambers, the higher pressure exerts a force that moves the diaphragm toward the lower-pressure area
  • Vacuum Booster Theory
    • The greater the pressure differential, the greater the force
    • To calculate the force, the pressure differential is multiplied by the diaphragm surface area
  • Figure 105-6 Vacuum brake boosters operate on the principle of pressure differential.
  • CHARCOAL FILTER
  • Charcoal Filter
    • If a dip or sag occurs in the vacuum hose leading from the engine to the power boost, condensed fuel vapors and/or moisture can accumulate and can block or restrict the vacuum to the booster
  • Charcoal Filter
    • Many manufacturers use a small charcoal filter in the vacuum line between the engine and booster to attract and hold gasoline vapors and keep fumes from entering the vacuum booster
  • Figure 105-7 The charcoal filter traps gasoline vapors that are present in the intake manifold and prevents them from getting into the vacuum chamber of the booster.
  • VACUUM CHECK VALVE
  • Vacuum Check Valve
    • All vacuum boosters use a one-way vacuum check valve
    • This valve allows air to flow in only one direction—from the booster toward the engine
  • Vacuum Check Valve
    • This valve prevents loss of vacuum when the engine stops
    • Without this check valve, the vacuum stored in the vacuum booster would simply be lost through the hose and intake manifold of the engine
  • Vacuum Check Valve
    • CAUTION: Sometimes an engine backfire can destroy or blow the vacuum check valve out of the booster housing. If this occurs, all power assist will be lost and a much greater-than- normal force must be exerted on the brake pedal to stop the vehicle.
  • Vacuum Check Valve
    • CAUTION: Be sure to repair the cause of the backfire before replacing the damaged or missing check valve. Normal causes of backfire include an excessively lean air–fuel ratio or incorrect firing order or ignition timing.
  • Figure 105-8 (a) Many vacuum brake booster check valves are located where the vacuum hose from the engine (vacuum source) attaches to the vacuum booster.
  • Figure 105-8 (b) This one-way valve prevents the loss of vacuum when the engine is off. The diaphragm inside allows air to flow in one direction only.
  • VACUUM BRAKE BOOSTER OPERATION
  • Vacuum Brake Booster Operation
    • A vacuum power-brake booster contains a rubber diaphragm(s) connected to the brake pedal at one end and to the master cylinder at the other end
    • When the brakes are off or released, there is equal vacuum on both sides of the diaphragm
  • Vacuum Brake Booster Operation
    • The vacuum power unit contains the power-piston assembly, which houses the control valve and reaction mechanism, and the power-piston return spring
  • Vacuum Brake Booster Operation
    • The control valve is composed of the air valve (valve plunger), the floating control-valve assembly, and the pushrod
  • Vacuum Brake Booster Operation
    • The reaction mechanism consists of a hydraulic piston reaction plate and a series of reaction levers
    • An air filter, air silencer, and filter retainer are assembled around the valve operating rod, filling the cavity inside the hub of the power piston
    • The pushrod that operates the air valve projects out of the end
  • Figure 105-10 Cross-sectional view of a typical vacuum brake booster assembly.
  • Vacuum Brake Booster Operation
    • Released-Position Operation
      • At the released position (brake pedal up), the air valve is seated on the floating control valve, which shuts off the air
  • Vacuum Brake Booster Operation
    • Released-Position Operation
      • The control valve is held away from the valve seat in the power-piston insert
      • Vacuum from the engine is present in the space on both sides of the power piston
  • Vacuum Brake Booster Operation
    • Released-Position Operation
      • Any air in the system is drawn through a small passage in the power piston, over the seat in the power-piston insert, and through a passage in the power-piston insert
  • Vacuum Brake Booster Operation
    • Released-Position Operation
      • Vacuum on both sides of the power piston is held against the rear of the housing by the power-piston return spring
  • Vacuum Brake Booster Operation
    • Released-Position Operation
      • At rest, the hydraulic reaction plate is held against the reaction retainer
  • Vacuum Brake Booster Operation
    • Released-Position Operation
      • The air-valve spring holds the reaction lever against the hydraulic reaction plate and holds the air valve against its stop in the tube of the power piston
  • Vacuum Brake Booster Operation
    • Released-Position Operation
      • The floating control-valve assembly is held against the air-valve seat by the floating control-valve spring
  • Figure 105-11 In the release position (brake pedal up), the vacuum is directed to both sides of the diaphragm.
  • Vacuum Brake Booster Operation
    • Applied-Position Operation
      • As the brake pedal is depressed, the floating control valve is moved toward its seat in the power piston
  • Vacuum Brake Booster Operation
    • Applied-Position Operation
      • The smaller air valve spring causes the air valve to stretch out until it reaches the lip of the power piston’s vacuum passage, closing off the vacuum supply to the rear section of the housing
  • Vacuum Brake Booster Operation
    • Applied-Position Operation
      • Atmospheric air enters between the air valve and the control valve pressurizing the rear section of the housing, while the front section is under vacuum
  • Vacuum Brake Booster Operation
    • Applied-Position Operation
      • Atmospheric pressure can then force the power piston forward
  • Vacuum Brake Booster Operation
    • NOTE: This movement of air into the rear chamber of the brake booster may be heard inside the vehicle as a hissing noise. The loudness of this airflow varies from vehicle to vehicle and should be considered normal.
  • Vacuum Brake Booster Operation
    • Applied-Position Operation
      • As the power piston travels forward, the master cylinder primary and secondary pistons are pushed forward
  • Vacuum Brake Booster Operation
    • Applied-Position Operation
      • As back-pressure builds up on the end of the master cylinder piston, the control valve is pushed off of its seat, applying back-pressure to the brake pedal
  • Vacuum Brake Booster Operation
    • Applied-Position Operation
      • The power piston return spring generates some brake pedal force
      • Approximately 30% of the brake load is applied back to the brake pedal
  • Figure 105-12 Simplified diagram of a vacuum brake booster in the apply position. Notice that the atmospheric valve is open and air pressure is being applied to the diaphragm.
  • Vacuum Brake Booster Operation
    • Hold-Position Operation
      • When the desired brake pedal force is reached and there is balance between the opposing forces of the brake pedal and the master cylinder, the power piston moves forward “around” the floating control valve and reaction disc until the air valve sealing end “catches up” with the floating control valve
  • Vacuum Brake Booster Operation
    • Hold-Position Operation
      • The air valve is sealed against the floating control valve and is not blocking the vacuum passage in the power piston
      • The floating control valve is held away from its seat
  • Vacuum Brake Booster Operation
    • Hold-Position Operation
      • There is vacuum on both sides of the diaphragm and power piston
      • Brake pedal force is keeping the power piston in its position
  • Vacuum Brake Booster Operation
    • Hold-Position Operation
      • If additional braking is required, the floating control valve moves away from the air valve permitting the power of atmospheric pressure to push the power piston and master cylinder pistons forward
  • Vacuum Brake Booster Operation
    • Hold-Position Operation
      • If the pedal is released, the power piston return spring moves the power piston to its released state
  • Figure 105-13 Cross section of a vacuum brake booster in the hold position with both vacuum and atmospheric valves closed. Note that the reaction force from the brake fluid pressure is transferred back to the driver as a reaction force to the brake pedal.
  • Vacuum Brake Booster Operation
    • Vacuum-Failure Mode
      • In case of vacuum failure, the brake operates as follows:
        • As the pedal is pushed down, the operating rod forces the floating control valve against the power piston and reaction disc
  • Vacuum Brake Booster Operation
    • Vacuum-Failure Mode
      • In case of vacuum failure, the brake operates as follows:
        • This force is then applied to the pushrod and the hydraulic reaction plate fastened to the master cylinder piston rod and pressure is applied in the master cylinder
  • Vacuum Brake Booster Operation
    • Vacuum-Failure Mode
      • For safety in the event of a stalled engine and a loss of vacuum, a power brake booster should have adequate storage of vacuum for several power-assisted stops
  • DUAL- (TANDEM-) DIAPHRAGM VACUUM BOOSTERS
  • Dual- (Tandem-) Diaphragm Vacuum Boosters
    • Instead of increasing the diameter of a single diaphragm, two smaller-diameter diaphragms are placed one in front of the other, increasing the total area without increasing the physical diameter of the booster
  • Figure 105-14 Cutaway showing a dual-diaphragm (tandem) vacuum brake booster.
  • BRAKE ASSIST SYSTEM
  • Brake Assist System
    • Some vehicles are equipped with a brake assist system (BAS) that applies the brakes with maximum force if the system detects that the driver is making a panic stop
  • Brake Assist System
    • Operation
      • The brake assist system opens an air valve on the rear part of the vacuum booster assembly so that more air at atmospheric pressure can flow into the rear chamber of the vacuum booster, increasing the force applied to the master cylinder
  • Brake Assist System
    • Operation
      • The BAS function works with the electronic stability control (ESC) system to ensure maximum braking efficiency during evasive or emergency situations
  • Brake Assist System
    • Operation
      • If the speed of the brake pedal application exceeds a predetermined limit according to the brake pedal travel sensor, the ABS controller energizes the BAS solenoid valve
  • Brake Assist System
    • Operation
      • The solenoid valve opens and additional air at atmospheric pressure enters the driver’s side of the booster and applies the brakes faster and with more force
  • Brake Assist System
    • Operation
      • The BAS solenoid is de-energized when the brake pedal is released and normal braking returns
  • Figure 105-16 When the brake assist function operates, the brake force is much higher than normal.
  • Figure 105-17 Typical adjustable pushrod. This adjustment is critical for the proper operation of the braking system. If the pushrod is too long, the brakes may be partially applied during driving. If the rod is too short, the brake pedal may have to be depressed farther down before the brakes start to work.
  • VACUUM BOOSTER OPERATION TEST
  • Vacuum Booster Operation Test
    • With the engine “off,” apply the brakes several times to deplete the vacuum
    • With your foot on the brake pedal, start the engine
  • Vacuum Booster Operation Test
    • The brake pedal should drop
    • If the brake pedal does not drop, check for proper vacuum source to the booster
    • There should be at least 15 in. Hg of vacuum for proper operation
  • Vacuum Booster Operation Test
    • If there is proper vacuum, repair or replacement of the power booster is required
  • VACUUM BOOSTER LEAK TEST
  • Vacuum Booster Leak Test
    • To test if the vacuum booster can hold a vacuum perform the following steps:
      • STEP 1: Operate the engine to build up a vacuum in the booster, then turn the engine off.
  • Vacuum Booster Leak Test
    • To test if the vacuum booster can hold a vacuum perform the following steps:
      • STEP 2: Wait one minute.
  • Vacuum Booster Leak Test
    • To test if the vacuum booster can hold a vacuum perform the following steps:
      • STEP 3: Depress the brake pedal several times. There should be two or more power-assisted brake applications.
  • Vacuum Booster Leak Test
    • If applications are not power assisted, either the vacuum check valve or the booster is leaking
  • Vacuum Booster Leak Test
    • To test the check valve, remove the valve from the booster and blow through the check valve
    • If air passes through, the valve is defective and must be replaced
  • Vacuum Booster Leak Test
    • If the check valve is okay, the vacuum booster is leaking and should be repaired or replaced
  • HYDRAULIC SYSTEM LEAK TEST
  • Hydraulic System Leak Test
    • To test if the hydraulic system (and not the booster) is leaking, depress and release the brake pedal (service brakes) several times to deplete the power-assist
  • Hydraulic System Leak Test
    • Depress and hold the brake pedal with medium force (20 to 35 lb or 88 to 154 N)
    • If the pedal falls, the hydraulic brake system is leaking
  • Hydraulic System Leak Test
    • Check for external leakage at wheel cylinders, calipers, hydraulic lines, and hoses
    • If there is no external leak, there may be an internal leak inside the master cylinder
  • PUSHROD CLEARANCE ADJUSTMENT
  • Pushrod Clearance Adjustment
    • Check the pushrod length whenever the vacuum brake booster or the master cylinder is replaced
    • The length of the pushrod must match correctly with the master cylinder
  • Pushrod Clearance Adjustment
    • If the pushrod is too long and the master cylinder is installed, the rod may be applying a force on the primary piston, causing the brakes to overheat and the brake fluid to boil
  • Pushrod Clearance Adjustment
    • If the brake fluid boils, a total loss of braking force can occur
    • A gauge is often used to measure the position of the master cylinder piston, and then the other end of the gauge is used to determine the proper pushrod clearance
  • Figure 105-16 When the brake assist function operates, the brake force is much higher than normal.
  • Figure 105-17 Typical adjustable pushrod. This adjustment is critical for the proper operation of the braking system. If the pushrod is too long, the brakes may be partially applied during driving. If the rod is too short, the brake pedal may have to be depressed farther down before the brakes start to work.
  • Figure 105-18 (a) Typical vacuum brake booster pushrod gauging tool. (a) The tool is first placed against the mounting flange of the master cylinder and the depth of the piston determined.
  • Figure 105-18 (b) Typical vacuum brake booster pushrod gauging tool. (b) The gauge is then turned upside down and used to gauge the pushrod length. Some vacuum brake boosters do not use adjustable pushrods. If found to be the incorrect length, a replacement pushrod of the correct length should be installed.
  • VACUUM BOOSTER DISASSEMBLY AND SERVICE
  • Vacuum Booster Disassembly and Service
    • Some manufacturers recommend that the vacuum brake booster be replaced as an assembly if leaking or defective and some recommend that it be disassembled and overhauled
  • Vacuum Booster Disassembly and Service
    • A special holding fixture should be used before rotating (unlocking) the front and rear housing because the return spring is strong
  • Vacuum Booster Disassembly and Service
    • Disassemble the vacuum brake booster according to the manufacturer’s recommended procedures for the specific unit being serviced
    • A rebuilding kit is available that includes all necessary parts and the proper silicone grease
    ?
  • Figure 105-19 A holding fixture and a long tool being used to rotate the two halves of a typical vacuum brake booster.
  • Figure 105-20 Exploded view of a typical dual-diaphragm vacuum brake booster assembly.
  • HYDRO-BOOST HYDRAULIC BRAKE BOOSTER
  • Hydro-Boost Hydraulic Brake Booster
    • The Hydro-Boost system uses the pressurized hydraulic fluid from the vehicle’s power steering pump as a power source
    • The Hydro-Boost unit is used on vehicles that lack enough engine vacuum, such as turbocharged or diesel engine vehicles
  • Hydro-Boost Hydraulic Brake Booster
    • During operation, diesel engines do not produce vacuum in the intake manifold, so they must use accessory engine-driven vacuum pumps to operate vacuum accessories
  • Hydro-Boost Hydraulic Brake Booster
    • Turbocharged and supercharged engines do not create engine vacuum during periods of acceleration
    • Even though vacuum is available when the engine is decelerating, some vehicle manufacturers elect to install a Hydro-Boost system
  • Figure 105-21 Hydro-Boost unit attaches between the bulkhead and the master cylinder and is powered by the power steering pump.
  • Figure 105-22 Exploded view of the Hydro-Boost unit.
  • Hydro-Boost Hydraulic Brake Booster
    • Operation
      • Fluid pressure from the power steering pump enters the unit and is directed by a spool valve
      • When the brake pedal is depressed, the lever and primary valve are moved
  • Hydro-Boost Hydraulic Brake Booster
    • Operation
      • The valve closes off the return port, causing pressure to build in the boost pressure chamber
      • The hydraulic pressure pushes on the power piston, which then applies force to the output rod that connects to the master cylinder piston
  • Hydro-Boost Hydraulic Brake Booster
    • Operation
      • In the event of a power steering pump failure, power assist is still available for several brake applications
      • During operation, hydraulic fluid under pressure from the power steering pump pressurizes an accumulator
  • Hydro-Boost Hydraulic Brake Booster
    • Operation
      • The fluid trapped in the accumulator under pressure is used to provide power-assisted stops in the event of a hydraulic system failure
  • Figure 105-23 A Hydro-Boost hydraulic booster in the unapplied position.
  • Figure 105-24 A Hydro-Boost hydraulic booster as the brakes are applied.
  • Figure 105-25 A Hydro-Boost hydraulic booster in the holding position.
  • Hydro-Boost Hydraulic Brake Booster
    • Diagnosis
      • The first step is to perform a thorough visual inspection, including the following:
        • Checking for proper power steering fluid level.
  • Hydro-Boost Hydraulic Brake Booster
    • Diagnosis
      • The first step is to perform a thorough visual inspection, including the following:
        • Checking for leaks from the unit or power steering pump.
  • Hydro-Boost Hydraulic Brake Booster
    • Diagnosis
      • The first step is to perform a thorough visual inspection, including the following:
        • Checking the condition and tightness of the power steering drive belt.
  • Hydro-Boost Hydraulic Brake Booster
    • Diagnosis
      • The first step is to perform a thorough visual inspection, including the following:
        • Checking for proper operation of the base brake system.
  • Hydro-Boost Hydraulic Brake Booster
    • Diagnosis
      • After checking all of the visual components, check for proper pressure and volume from the power steering pump
  • Hydro-Boost Hydraulic Brake Booster
    • Diagnosis
      • The pump should be able to produce a minimum of 2 gallons (7.5 liters) with a maximum pressure of 150 PSI (1,000 kPa) with the steering in the straight-ahead position
  • Hydro-Boost Hydraulic Brake Booster
    • Diagnosis
      • With the engine “off", the accumulator should be able to supply a minimum of two power-assisted brake applications
  • Figure 105-26 A typical Hydro-Boost hydraulic line arrangement showing the pump, steering gear, and brake booster assembly.
  • Figure 105-27 Pressure and flow analyzer installation to check the power steering pump output.
  • Hydro-Boost Hydraulic Brake Booster
    • Hydro-Boost Function Test
      • With the engine off, apply the brake pedal several times until the accumulator is depleted completely
  • Hydro-Boost Hydraulic Brake Booster
    • Hydro-Boost Function Test
      • Depress the service brake pedal and start the engine
      • The pedal should fall and then push back against the driver’s foot
  • HYDRO-BOOST SYMPTOM-BASED GUIDE
  • Hydro-Boost Symptom-Based Guide
    • Excessive Brake Pedal Effort
      • Possible causes for this include the following:
        • Loose or broken power steering pump belt
  • Hydro-Boost Symptom-Based Guide
    • Excessive Brake Pedal Effort
      • Possible causes for this include the following:
        • No fluid in the power steering reservoir
  • Hydro-Boost Symptom-Based Guide
    • Excessive Brake Pedal Effort
      • Possible causes for this include the following:
        • Leaks in the power steering, booster, or accumulator hoses
  • Hydro-Boost Symptom-Based Guide
    • Excessive Brake Pedal Effort
      • Possible causes for this include the following:
        • Leaks at tube fittings, power steering, booster, or accumulator connections
  • Hydro-Boost Symptom-Based Guide
    • Excessive Brake Pedal Effort
      • Possible causes for this include the following:
        • External leakage at the accumulator
  • Hydro-Boost Symptom-Based Guide
    • Excessive Brake Pedal Effort
      • Possible causes for this include the following:
        • Faulty booster piston seal, causing leakage at the booster flange vent
  • Hydro-Boost Symptom-Based Guide
    • Excessive Brake Pedal Effort
      • Possible causes for this include the following:
        • Faulty booster cover seal with leakage between the housing and cover
  • Hydro-Boost Symptom-Based Guide
    • Excessive Brake Pedal Effort
      • Possible causes for this include the following:
        • Faulty booster spool plug seal
  • Hydro-Boost Symptom-Based Guide
    • Slow Brake Pedal Return
      • Possible causes for this include the following:
        • Excessive seal friction in the booster
  • Hydro-Boost Symptom-Based Guide
    • Slow Brake Pedal Return
      • Possible causes for this include the following:
        • Faulty spool action
  • Hydro-Boost Symptom-Based Guide
    • Slow Brake Pedal Return
      • Possible causes for this include the following:
        • Broken piston return spring
  • Hydro-Boost Symptom-Based Guide
    • Slow Brake Pedal Return
      • Possible causes for this include the following:
        • 4. Restriction in the return line from the booster to the pump reservoir
  • Hydro-Boost Symptom-Based Guide
    • Slow Brake Pedal Return
      • Possible causes for this include the following:
        • 5. Broken spool return spring
  • Hydro-Boost Symptom-Based Guide
    • Grabby Brakes
      • Possible causes for this include the following:
        • Broken spool return spring
  • Hydro-Boost Symptom-Based Guide
    • Grabby Brakes
      • Possible causes for this include the following:
        • Faulty spool action caused by contamination in the system
  • Hydro-Boost Symptom-Based Guide
    • Booster Chatters – Pedal Vibrates
      • Possible causes for this include the following:
        • Power steering pump belt slipping
  • Hydro-Boost Symptom-Based Guide
    • Booster Chatters – Pedal Vibrates
      • Possible causes for this include the following:
        • Low fluid level in the power steering pump reservoir
  • Hydro-Boost Symptom-Based Guide
    • Booster Chatters – Pedal Vibrates
      • Possible causes for this include the following:
        • Faulty spool operation caused by contamination in the system
  • TECH TIP
    • Check the Vacuum, Then the Brakes
      • A customer complained of a very rough idle and an occasional pulsating brake pedal. The customer was certain that the engine required serious work since there were over 100,000 miles on the vehicle. During the troubleshooting procedure, a spray cleaner was used to find any vacuum (air) leaks.
    BACK TO PRESENTATION
    • A large hole was found melted through a large vacuum hose next to the vacuum hose feeding the vacuum-operated power brake booster.
    • After repairing the vacuum leak, the vehicle was test driven again to help diagnose the cause of the pulsating brake pedal. The engine idled very smoothly after the vacuum leak was repaired and the brake pulsation was also cured.
    • The vacuum leak resulted in lower-than-normal vacuum being applied to the vacuum booster. During braking, when engine vacuum is normally higher (deceleration), the vacuum booster would assist, then not assist when the vacuum was lost. This on-and-off supply of vacuum to the vacuum booster was noticed by the driver as a brake pulsation. Always check the vacuum at the booster whenever diagnosing any brake problems.
    • Most vehicle manufacturers specify a minimum of 15 in. Hg of vacuum at the booster. The booster should be able to provide at least two or three stops even with no vacuum. The booster should also be checked to see if it can hold a vacuum after several hours. A good vacuum booster, for example, should be able to provide a power assist after sitting all night without starting the engine.
  • TECH TIP
    • A Low, Soft Brake Pedal Is Not a Power Booster Problem
      • Some service technicians tend to blame the power brake booster if the vehicle has a low, soft brake pedal. A defective power brake booster causes a hard brake pedal, not a soft brake pedal. A soft or spongy brake pedal is usually caused by air being trapped somewhere in the hydraulic system.
    BACK TO PRESENTATION
    • Many times, the technician has bled the system and, therefore, thinks that the system is free of any trapped air. According to remanufacturers of master cylinders and power brake boosters, most of the returned parts under warranty are not defective. Incorrect or improper bleeding procedures account for much of the problem.
  • FREQUENTLY ASKED QUESTION
    • What Is Supplemental Brake Assist?
      • Supplemental brake assist, SBA, is a motor-driven vacuum pump that can supplement engine vacuum to the vacuum brake booster. This unit is used on some General Motors vehicles. When a vehicle is driven under a heavy load, engine vacuum is low.
    ? BACK TO PRESENTATION
    • To meet the brake standards, some vehicles are equipped with the brake assist system that consists of the following components:
    • A pressure sensor that is used to measure the vacuum in the vacuum booster.
    • An intake manifold check valve that is used to prevent vacuum from escaping the vacuum boost.
    • A motor-driven vacuum pump.
    • The vacuum pump motor will start and run if the pressure sensor detects the vacuum in the booster is below 7 in. Hg and will shut off after the vacuum level increases to 9 in. Hg.
  • TECH TIP
    • The Hydro-Boost Accumulator Test
      • The accumulator stores hydraulic fluid under pressure to provide a reserve in the event of a failure of the power steering system. The accumulator is designed to provide three or more power-assisted stops with the engine off.
    BACK TO PRESENTATION
    • If the accumulator fails, it does not hold pressure. To easily check whether the accumulator has lost its charge, simply grasp the accumulator with your hand and try to twist or move it. The accumulator should have so much pressure on it that it should not move or wiggle. If the accumulator moves, it has lost its ability to hold pressure and the Hydro-Boost unit should be replaced.
      • Figure 105-28 The accumulator should be able to hold pressure and feel tight when hand force is used to try to move it.