halderman ch105 lecture

© 2011 Pearson Education, Inc.All Rights Reserved

Automotive Technology, Fourth EditionJames Halderman

POWER BRAKE UNIT OPERATION,

DIAGNOSIS, AND SERVICE

105

105 POWER BRAKE UNIT OPERATION, DIAGNOSIS, AND SERVICE



ObjectivesObjectives• 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.




ObjectivesObjectives• The student should be able to:

– Explain how to test a vacuum brake booster.

– Describe how a hydraulic brake booster operates.




THE NEED FOR POWERTHE NEED FOR POWERBRAKE ASSISTBRAKE ASSIST




The Need for Power Brake AssistThe 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 AssistThe 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 PRINCIPLES OF VACUUMVACUUM




Principles of VacuumPrinciples 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 VacuumPrinciples 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 VacuumPrinciples 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 VacuumPrinciples of Vacuum• Measuring Vacuum

– Vacuum is measured in inches of mercury (in. Hg) or in millimeters of mercury (mm Hg)





– Vacuum is a measurement of the pressure differential between the lower pressure inside the tube, and the higher pressure outside





– A perfect vacuum is about 30 in. Hg (762 mm Hg) and is never achieved in an engine’s intake manifold





– Manifold vacuum varies with throttle position

– The lowest manifold vacuum (highest pressure) occurs when the throttle is wide open with the engine under load





– 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





– 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 VacuumPrinciples 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





– 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





– 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 VACUUM BOOSTER THEORYTHEORY




Vacuum Booster TheoryVacuum 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 TheoryVacuum 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 TheoryVacuum 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 FILTERCHARCOAL FILTER




Charcoal FilterCharcoal 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 FilterCharcoal 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 VALVEVACUUM CHECK VALVE




Vacuum Check ValveVacuum 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 ValveVacuum 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 ValveVacuum 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 ValveVacuum 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 BRAKEVACUUM BRAKEBOOSTER OPERATIONBOOSTER OPERATION




Vacuum Brake Booster OperationVacuum 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 OperationVacuum 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 OperationVacuum 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 OperationVacuum 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 OperationVacuum 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





– 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





– 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 on both sides of the power piston is held against the rear of the housing by the power-piston return spring





– At rest, the hydraulic reaction plate is held against the reaction retainer





– 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





– 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 OperationVacuum 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





– 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





– 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





– Atmospheric pressure can then force the power piston forward




Vacuum Brake Booster OperationVacuum 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.





– As the power piston travels forward, the master cylinder primary and secondary pistons are pushed forward





– 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





– 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 OperationVacuum 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





– 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





– There is vacuum on both sides of the diaphragm and power piston

– Brake pedal force is keeping the power piston in its position





– 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





– 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 OperationVacuum 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





– 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





– 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-)DUAL- (TANDEM-)DIAPHRAGM VACUUMDIAPHRAGM VACUUM

BOOSTERSBOOSTERS




Dual- (Tandem-) Diaphragm Vacuum Dual- (Tandem-) Diaphragm Vacuum BoostersBoosters

• 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 SYSTEMBRAKE ASSIST SYSTEM




Brake Assist SystemBrake 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 SystemBrake 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





– The BAS function works with the electronic stability control (ESC) system to ensure maximum braking efficiency during evasive or emergency situations





– 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





– 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





– 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 BOOSTERVACUUM BOOSTEROPERATION TESTOPERATION TEST




Vacuum Booster Operation TestVacuum 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 TestVacuum 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 TestVacuum Booster Operation Test• If there is proper vacuum, repair or

replacement of the power booster is required




VACUUM BOOSTER VACUUM BOOSTER LEAK TESTLEAK TEST




Vacuum Booster Leak TestVacuum 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.





a vacuum perform the following steps:– STEP 2: Wait one minute.





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 TestVacuum Booster Leak Test• If applications are not power assisted,

either the vacuum check valve or the booster is leaking




Vacuum Booster Leak TestVacuum 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 TestVacuum Booster Leak Test• If the check valve is okay, the vacuum

booster is leaking and should be repaired or replaced




HYDRAULIC SYSTEMHYDRAULIC SYSTEMLEAK TESTLEAK TEST




Hydraulic System Leak TestHydraulic 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 TestHydraulic 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 TestHydraulic 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 CLEARANCEPUSHROD CLEARANCEADJUSTMENTADJUSTMENT




Pushrod Clearance AdjustmentPushrod 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 AdjustmentPushrod 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 AdjustmentPushrod 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 BOOSTERVACUUM BOOSTERDISASSEMBLY AND DISASSEMBLY AND

SERVICESERVICE




Vacuum Booster Disassembly and Vacuum Booster Disassembly and Service 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





• A special holding fixture should be used before rotating (unlocking) the front and rear housing because the return spring is strong





• 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 HYDRO-BOOST HYDRAULICHYDRAULIC

BRAKE BOOSTERBRAKE BOOSTER




Hydro-Boost Hydraulic Brake BoosterHydro-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 BoosterHydro-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 BoosterHydro-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 BoosterHydro-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





– 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





– 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





– 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 BoosterHydro-Boost Hydraulic Brake Booster• Diagnosis

– The first step is to perform a thorough visual inspection, including the following:1. Checking for proper power steering fluid

level.





– The first step is to perform a thorough visual inspection, including the following:2. Checking for leaks from the unit or power

steering pump.





– The first step is to perform a thorough visual inspection, including the following:3. Checking the condition and tightness of

the power steering drive belt.





– The first step is to perform a thorough visual inspection, including the following:4. Checking for proper operation of the base

brake system.





– After checking all of the visual components, check for proper pressure and volume from the power steering pump





– 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





– 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 BoosterHydro-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 BoosterHydro-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-BOOSTHYDRO-BOOSTSYMPTOM-BASED SYMPTOM-BASED

GUIDEGUIDE




Hydro-Boost Symptom-Based GuideHydro-Boost Symptom-Based Guide• Excessive Brake Pedal Effort

– Possible causes for this include the following:1. Loose or broken power steering pump belt





– Possible causes for this include the following:2. No fluid in the power steering reservoir





– Possible causes for this include the following:3. Leaks in the power steering, booster, or

accumulator hoses





– Possible causes for this include the following:4. Leaks at tube fittings, power steering,

booster, or accumulator connections





– Possible causes for this include the following:5. External leakage at the accumulator





– Possible causes for this include the following:6. Faulty booster piston seal, causing leakage

at the booster flange vent





– Possible causes for this include the following:7. Faulty booster cover seal with leakage

between the housing and cover





– Possible causes for this include the following:8. Faulty booster spool plug seal




Hydro-Boost Symptom-Based GuideHydro-Boost Symptom-Based Guide• Slow Brake Pedal Return

– Possible causes for this include the following:1. Excessive seal friction in the booster





– Possible causes for this include the following:2. Faulty spool action





– Possible causes for this include the following:3. Broken piston return spring





– Possible causes for this include the following:

• 4. Restriction in the return line from the booster to the pump reservoir





– Possible causes for this include the following:

• 5. Broken spool return spring




Hydro-Boost Symptom-Based GuideHydro-Boost Symptom-Based Guide• Grabby Brakes

– Possible causes for this include the following:1. Broken spool return spring




Hydro-Boost Symptom-Based GuideHydro-Boost Symptom-Based Guide• Grabby Brakes

– Possible causes for this include the following:2. Faulty spool action caused by

contamination in the system




Hydro-Boost Symptom-Based GuideHydro-Boost Symptom-Based Guide• Booster Chatters – Pedal Vibrates

– Possible causes for this include the following:1. Power steering pump belt slipping





– Possible causes for this include the following:2. Low fluid level in the power steering pump

reservoir





– Possible causes for this include the following:3. Faulty spool operation caused by

contamination in the system

halderman ch105 lecture

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