CHAPTER 16 Power Brake Unit Operation Diagnosis and

CHAPTER 16 Power Brake Unit Operation, Diagnosis, and Service

OBJECTIVES After studying Chapter 16, the reader will be able to: 1. 2. 3. 4. 5. Prepare for the Brakes (A 5) 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. Explain how to test a vacuum brake booster. Describe how a hydraulic or electrohydraulic brake booster operates. Automotive Brake Systems, 5/e By James D. Halderman 2 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

KEY TERMS • • • Atmospheric pressure Brake assist system (BAS) Dual-diaphragm vacuum booster in. Hg (inches of mercury) mm Hg (millimeters of mercury) Automotive Brake Systems, 5/e By James D. Halderman • • • 3 Power chamber Pressure differential supplemental brake assist (SBA) Tandem-diaphragm vacuum booster Vacuum Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

THE NEED FOR POWER BRAKE ASSIST • • • To double the stopping power of a disc brake, the driver must double the force on the brake pedal. This is the reason that most vehicles equipped with disc brakes are power assisted, even on small, lightweight vehicles. The use of semimetallic brake pads also requires greater force. The most commonly used powerassisted units are vacuum operated. When a power booster is fitted, the brake pedal ratio is decreased and the master cylinder bore size is increased. The combined effect of these changes is to reduce pedal effort, while greatly increasing pedal reserve. Automotive Brake Systems, 5/e By James D. Halderman 4 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

THE NEED FOR POWER BRAKE ASSIST FIGURE 16– 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. Automotive Brake Systems, 5/e By James D. Halderman 5 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

THE NEED FOR POWER BRAKE ASSIST FIGURE 16– 2 A wide brake pedal allows two-foot braking if power assist is lost. Automotive Brake Systems, 5/e By James D. Halderman 6 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

PRINCIPLES OF VACUUM • • Most vacuum-powered brake boosters get their vacuum supply from the engine intake manifold. An engine is essentially a big air pump because the pistons move up and down in the cylinders to pump in air and fuel, and pump out exhaust. They do this by creating differences in air pressure. As a piston moves downward on an intake stroke with the intake valve open, it creates a larger area inside the cylinder for air to fill. Automotive Brake Systems, 5/e By James D. Halderman 7 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

PRINCIPLES OF VACUUM • • • This 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. Although it may seem as though the low pressure is pulling air into the engine, it is really the higher pressure outside that forces air in. The difference in pressure between two areas is called a pressure differential. Automotive Brake Systems, 5/e By James D. Halderman 8 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

PRINCIPLES OF VACUUM FIGURE 16– 3 Atmospheric pressure varies with altitude. Automotive Brake Systems, 5/e By James D. Halderman 9 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

PRINCIPLES OF VACUUM MEASURING VACUUM • • Vacuum is measured in inches of mercury (in. Hg) or in millimeters of mercury (mm Hg), a figure that indicates how far a column of mercury in a tube will rise when a vacuum is applied at one end, and atmospheric pressure at the other. Vacuum is a measurement of the pressure differential between the lower pressure inside the tube, and the higher pressure outside it. Automotive Brake Systems, 5/e By James D. Halderman 10 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

PRINCIPLES OF VACUUM BOOSTER VACUUM SUPPLY • • • Vacuum boosters get their vacuum supply from the engine intake manifold. Diesel engines, however, run unthrottled (engine speed is controlled strictly by the amount of fuel injected) 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. Automotive Brake Systems, 5/e By James D. Halderman 11 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

PRINCIPLES OF VACUUM BOOSTER VACUUM SUPPLY FIGURE 16– 4 A belt-driven auxiliary vacuum pump. Automotive Brake Systems, 5/e By James D. Halderman 12 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

PRINCIPLES OF VACUUM BOOSTER VACUUM SUPPLY FIGURE 16– 5 An electrically powered vacuum pump. Automotive Brake Systems, 5/e By James D. Halderman 13 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

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. 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. Rods attached to the diaphragm transmit this force, plus the force the driver exerts on the brake pedal, to the master cylinder. Automotive Brake Systems, 5/e By James D. Halderman 14 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

VACUUM BOOSTER THEORY FIGURE 16– 6 Vacuum brake boosters operate on the principle of pressure differential. Automotive Brake Systems, 5/e By James D. Halderman 15 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

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. A large hole was found melted through a large vacuum hose next to the vacuum hose feeding the vacuum-operated power brake booster. Automotive Brake Systems, 5/e By James D. Halderman 16 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

Check the Vacuum, Then the Brakes • 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 maximum 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. Automotive Brake Systems, 5/e By James D. Halderman 17 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

CHARCOAL FILTER • • • The vacuum hose leading from the engine to the power booster should run downward without any low places in the hose. If a dip or sag occurs in the vacuum hose, condensed fuel vapors and/or moisture can accumulate that can block or restrict the vacuum to the booster. Many manufacturers use a small charcoal filter in the vacuum line between the engine and booster. Automotive Brake Systems, 5/e By James D. Halderman 18 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

CHARCOAL FILTER FIGURE 16– 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. Automotive Brake Systems, 5/e By James D. Halderman 19 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

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. 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. Automotive Brake Systems, 5/e By James D. Halderman 20 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

VACUUM CHECK VALVE FIGURE 16– 8 (a) Many vacuum brake booster check valves are located where the vacuum hose from the engine (vacuum source) attaches to the vacuum booster. (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. Automotive Brake Systems, 5/e By James D. Halderman 21 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

VACUUM CHECK VALVE FIGURE 16– 9 Not all check valves are located at the vacuum line to the booster housing connection. This vehicle uses an inline check valve located between the intake manifold of the engine and the vacuum brake booster. Automotive Brake Systems, 5/e By James D. Halderman 22 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

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. The vacuum power unit contains the power-piston assembly, which houses the control valve and reaction mechanism, and the power-piston return spring. Automotive Brake Systems, 5/e By James D. Halderman 23 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

VACUUM BRAKE BOOSTER OPERATION FIGURE 16– 10 Cross-sectional view of a typical vacuum brake booster assembly. Automotive Brake Systems, 5/e By James D. Halderman 24 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

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. The floating control valve is held away from the valve seat in the power-piston insert. FIGURE 16– 11 In the release position (brake pedal up), the vacuum is directed to both sides of the diaphragm. Automotive Brake Systems, 5/e By James D. Halderman 25 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

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, away from the rear of the booster. The smaller air valve spring causes the air valve to stretch out toward the retreating floating control valve until it bottoms out on the lip of the power piston’s vacuum passage. This closes off the vacuum supply to the rear section of the housing. Automotive Brake Systems, 5/e By James D. Halderman 26 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

VACUUM BRAKE BOOSTER OPERATION APPLIED-POSITION OPERATION • • • Since the floating control valve travels farther than the sealing end of the air valve, atmospheric air is allowed to enter between the air valve and the floating control valve pressurizing the rear section of the housing. At this point, the rear section of the housing is pressurized and the front section is under vacuum. Atmospheric pressure can then force the power piston forward. Automotive Brake Systems, 5/e By James D. Halderman 27 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

VACUUM BRAKE BOOSTER OPERATION APPLIED-POSITION OPERATION FIGURE 16– 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. Automotive Brake Systems, 5/e By James D. Halderman 28 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

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. At this point, the air valve is once again sealed against the floating control valve and is no longer blocking the vacuum passage in the power piston. The floating control valve is again held away from its seat. Automotive Brake Systems, 5/e By James D. Halderman 29 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

VACUUM BRAKE BOOSTER OPERATION HOLD-POSITION OPERATION FIGURE 16– 13 Cross section of as 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. Automotive Brake Systems, 5/e By James D. Halderman 30 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

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. 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. Automotive Brake Systems, 5/e By James D. Halderman 31 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

VACUUM BRAKE BOOSTER OPERATION VACUUM-FAILURE MODE • • In case of vacuum source interruption, the brake operates as a standard brake as follows. As the pedal is pushed down, the operating rod forces the floating control valve against the power piston and reaction disc. This force is then applied to the pushrod and subsequently the hydraulic reaction plate fastened to the master cylinder piston rod, which applies pressure 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. Automotive Brake Systems, 5/e By James D. Halderman 32 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

DUAL- (TANDEM-) DIAPHRAGM VACUUM BOOSTERS • • To provide power assist, air pressure must work against a rubber diaphragm. The larger the area of the diaphragm, the more force can be exerted. The usual method of increasing the area of the vacuum diaphragm was to increase the diameter of the vacuum booster. However, a larger vacuum booster took up too much room under the hood of many vehicles. Instead of increasing the diameter, vacuum booster manufacturers used two smaller-diameter diaphragms and placed one in front of the other. These designs increased the total area without increasing the physical diameter of the booster. This style is called a dualdiaphragm or tandemdiaphragm vacuum booster. Automotive Brake Systems, 5/e By James D. Halderman 33 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

DUAL- (TANDEM-) DIAPHRAGM VACUUM BOOSTERS FIGURE 16– 14 Cutaway showing a dual-diaphragm (tandem) vacuum brake booster. Automotive Brake Systems, 5/e By James D. Halderman 34 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

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. Tests performed by brake engineers have indicated that it is normal for a person to first apply the brakes rapidly during a panic situation. However, it was also found that the driver would tend to reduce the force applied to the brake pedal. As a result, the vehicle did not brake with the maximum effort. Automotive Brake Systems, 5/e By James D. Halderman 35 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

BRAKE ASSIST SYSTEM OPERATION • • • The brake assist system opens an air valve on the rear part of the vacuum booster assembly. As a result, more air at atmospheric pressure can flow into the rear chamber of the vacuum booster, thereby 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. Automotive Brake Systems, 5/e By James D. Halderman 36 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

BRAKE ASSIST SYSTEM OPERATION • • If the speed of the brake pedal application exceeds a predetermined limit as determined by the brake pedal travel sensor, the ABS controller energizes the BAS solenoid valve. When the solenoid valve opens, additional air at atmospheric pressure enters the driver’s side of the booster. The additional pressure applies the brakes faster and with more force. The BAS solenoid is deenergized when the brake pedal is released and normal braking returns. Automotive Brake Systems, 5/e By James D. Halderman 37 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

BRAKE ASSIST SYSTEM OPERATION FIGURE 16– 15 A typical brake assist system uses a brake pedal travel sensor and a BAS solenoid to apply the brakes during a panic condition. Automotive Brake Systems, 5/e By James D. Halderman 38 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

BRAKE ASSIST SYSTEM OPERATION FIGURE 16– 16 When the brake assist function operates, the brake force is much higher than normal. Automotive Brake Systems, 5/e By James D. Halderman 39 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

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. The brake pedal should drop. If the brake pedal does not drop, check for proper vacuum source to the booster. If there is proper vacuum, repair or replacement of the power booster is required. Automotive Brake Systems, 5/e By James D. Halderman 40 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

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. STEP 2 Wait one minute. STEP 3 Depress the brake pedal several times. There should be two or more power-assisted brake applications. Automotive Brake Systems, 5/e By James D. Halderman 41 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

HYDRAULIC SYSTEM LEAK TEST • • An internal or external hydraulic leak can also cause a brake system problem. To test if the hydraulic system (and not the booster) is leaking, depress and release the brake pedal (service brakes) several times. This should deplete any residual power assist. On some ABS units, this may require depressing the brake pedal 20 or more times! Automotive Brake Systems, 5/e By James D. Halderman 42 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

• http: //www. youtube. com/watch? v=Nk 6 e 5 t. TVsm 8 Automotive Brake Systems, 5/e By James D. Halderman 43 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

PUSHROD CLEARANCE ADJUSTMENT • • Whenever the vacuum brake booster or the master cylinder is replaced, the pushrod length should be checked. The length of the pushrod must match correctly with the master cylinder. FIGURE 16– 17 Typical adjustable pushrod. This adjustment is critical for the properation 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. Automotive Brake Systems, 5/e By James D. Halderman 44 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

PUSHROD CLEARANCE ADJUSTMENT FIGURE 16– 18 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. (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. Automotive Brake Systems, 5/e By James D. Halderman 45 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

VACUUM BOOSTER DISASSEMBLY AND SERVICE • • Some vehicle manufacturers recommend that the vacuum brake booster be disassembled and overhauled if defective. A special holding fixture should be used before rotating (unlocking) the front and rear housing because the return spring is strong. FIGURE 16– 19 A holding fixture and a long tool being used to rotate the two halves of a typical vacuum brake booster. Automotive Brake Systems, 5/e By James D. Halderman 46 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

VACUUM BOOSTER DISASSEMBLY AND SERVICE FIGURE 16– 20 Exploded view of a typical dual-diaphragm vacuum brake booster assembly. Automotive Brake Systems, 5/e By James D. Halderman 47 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

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. 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. Automotive Brake Systems, 5/e By James D. Halderman 48 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

HYDRO-BOOST HYDRAULIC BRAKE BOOSTER • • Hydro-Boost is a hydraulically operated powerassist unit built by Bendix. The Hydro-Boost system uses the pressurized hydraulic fluid from the vehicle’s power steering pump as a power source rather than using engine vacuum as is used with vacuum boosters. Automotive Brake Systems, 5/e By James D. Halderman 49 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

HYDRO-BOOST HYDRAULIC BRAKE BOOSTER FIGURE 16– 21 Hydro-Boost unit attaches between the bulkhead and the master cylinder and is powered by the power steering pump. (Courtesy of Allied Signal Automotive Aftermarket) Automotive Brake Systems, 5/e By James D. Halderman 50 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

HYDRO-BOOST HYDRAULIC BRAKE BOOSTER FIGURE 16– 22 Exploded view of the Hydro-Boost unit. (Courtesy of Allied Signal Automotive Aftermarket) Automotive Brake Systems, 5/e By James D. Halderman 51 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

HYDRO-BOOST HYDRAULIC BRAKE BOOSTER OPERATION FIGURE 16– 23 A Hydro-Boost hydraulic booster in the unapplied position. Automotive Brake Systems, 5/e By James D. Halderman 52 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

HYDRO-BOOST HYDRAULIC BRAKE BOOSTER OPERATION FIGURE 16– 24 A Hydro-Boost hydraulic booster as the brakes are applied. Automotive Brake Systems, 5/e By James D. Halderman 53 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

HYDRO-BOOST HYDRAULIC BRAKE BOOSTER OPERATION FIGURE 16– 25 A Hydro-Boost hydraulic booster in the holding position. Automotive Brake Systems, 5/e By James D. Halderman 54 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

HYDRO-BOOST HYDRAULIC BRAKE BOOSTER DIAGNOSIS • • The power source for Hydro-Boost units comes from the power steering pump. The first step of troubleshooting is to perform a thorough visual inspection, including the following: • • 1. Checking for proper power steering fluid level 2. Checking for leaks from the unit or power steering pump 3. Checking the condition and tightness of the power steering drive belt 4. Checking for properation of the base brake system Automotive Brake Systems, 5/e By James D. Halderman 55 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

HYDRO-BOOST HYDRAULIC BRAKE BOOSTER DIAGNOSIS FIGURE 16– 26 A typical Hydro-Boost hydraulic line arrangement showing the pump, steering gear, and brake booster assembly. Automotive Brake Systems, 5/e By James D. Halderman 56 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

HYDRO-BOOST HYDRAULIC BRAKE BOOSTER DIAGNOSIS FIGURE 16– 27 Pressure and flow analyzer installation to check the power steering pump output. Automotive Brake Systems, 5/e By James D. Halderman 57 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

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. 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 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. Automotive Brake Systems, 5/e By James D. Halderman 58 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

The Hydro-Boost Accumulator Test FIGURE 16– 28 The accumulator should be able to hold pressure and feel tight when hand force is used to try to move it. Automotive Brake Systems, 5/e By James D. Halderman 59 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

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. Depress the service brake pedal and start the engine. The pedal should fall and then push back against the driver’s foot. Automotive Brake Systems, 5/e By James D. Halderman 60 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

HYDRO-BOOST SYMPTOM-BASED GUIDE EXCESSIVE BRAKE PEDAL EFFORT • Possible causes for this include the following: • • 1. Loose or broken power steering pump belt 2. No fluid in the power steering reservoir 3. Leaks in the power steering, booster, or accumulator hoses 4. Leaks at tube fittings, power steering, booster, or accumulator connections 5. External leakage at the accumulator 6. Faulty booster piston seal, causing leakage at the booster flange vent 7. Faulty booster cover seal with leakage between the housing and cover 8. Faulty booster spool plug seal Automotive Brake Systems, 5/e By James D. Halderman 61 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

HYDRO-BOOST SYMPTOM-BASED GUIDE SLOW BRAKE PEDAL RETURN • Possible causes for this include the following: • • • 1. Excessive seal friction in the booster 2. Faulty spool action 3. Broken piston return spring 4. Restriction in the return line from the booster to the pump reservoir 5. Broken spool return spring Automotive Brake Systems, 5/e By James D. Halderman 62 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

HYDRO-BOOST SYMPTOM-BASED GUIDE GRABBY BRAKES • Possible causes for this include the following: • • 1. Broken spool return spring 2. Faulty spool action caused by contamination in the system Automotive Brake Systems, 5/e By James D. Halderman 63 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

HYDRO-BOOST SYMPTOM-BASED GUIDE BOOSTER CHATTERS—PEDAL VIBRATES • Possible causes for this include the following: • • • 1. Power steering pump belt slipping 2. Low fluid level in the power steering pump reservoir 3. Faulty spool operation caused by contamination in the system Automotive Brake Systems, 5/e By James D. Halderman 64 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

SUMMARY 1. 2. 3. Vacuum brake boosters use air pressure acting on a diaphragm to assist the driver’s force on the brake master cylinder. At rest, there is vacuum on both sides of the vacuum booster diaphragm. When the brake pedal is depressed, atmospheric air pressure is exerted on the back side of the diaphragm. The use of two diaphragms in tandem allows a smallerdiameter booster with the same area. The larger the area of the booster diaphragm, the more air pressure force can be applied to the master cylinder. Automotive Brake Systems, 5/e By James D. Halderman 65 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

SUMMARY 4. 5. 6. Hydraulic-operated brake boosters use either an electric motor-driven pump or the engine-driven power steering pump. When replacing a vacuum brake booster, always check for proper pushrod clearance. To be assured of power-assisted brake application in the event of failure, hydraulic power -assisted brake systems use an accumulator to provide pressure to the system. Automotive Brake Systems, 5/e By James D. Halderman 66 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

REVIEW QUESTIONS 1. 2. 3. 4. Describe the purpose and function of the one-way check valve used on vacuum brake booster units. Explain how vacuum is used to assist in applying the brakes. Describe how to perform a vacuum booster leak test and hydraulic system leak test. Explain how a Hydro-Boost system functions. Automotive Brake Systems, 5/e By James D. Halderman 67 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

CHAPTER QUIZ 1. Two technicians are discussing vacuum brake boosters. Technician A says that a low, soft brake pedal is an indication of a defective booster. Technician B says that there should be at least two power-assisted brake applications after the engine stops running. Which technician is correct? a. b. c. d. Technician A only Technician B only Both Technicians A and B Neither Technician A nor B Automotive Brake Systems, 5/e By James D. Halderman 68 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

CHAPTER QUIZ 2. Technician A says that to check the operation of a vacuum brake booster, the brake pedal should be depressed until the assist is depleted and then start the engine. Technician B says that the brake pedal should drop when the engine starts, if the booster is okay. Which technician is correct? a. b. c. d. Technician A only Technician B only Both Technicians A and B Neither Technician A nor B Automotive Brake Systems, 5/e By James D. Halderman 69 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

CHAPTER QUIZ 3. Brake pedal feedback to the driver is provided by the ____. a. b. c. d. Vacuum check-valve operation Reaction system Charcoal filter unit Vacuum diaphragm Automotive Brake Systems, 5/e By James D. Halderman 70 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

CHAPTER QUIZ 4. The properation of a vacuum brake booster requires that the engine be capable of supplying at least ____. a. b. c. d. 15 in. Hg vacuum 17 in. Hg vacuum 19 in. Hg vacuum 21 in. Hg vacuum Automotive Brake Systems, 5/e By James D. Halderman 71 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

CHAPTER QUIZ 5. The purpose of the charcoal filter in the vacuum hose between the engine and the vacuum brake booster is to ____. a. b. c. d. Filter the air entering the engine Trap gasoline vapors to keep them from entering the booster Act as a one-way check valve to help keep a vacuum reserve in the booster Direct the vacuum Automotive Brake Systems, 5/e By James D. Halderman 72 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

CHAPTER QUIZ 6. A defective vacuum brake booster will cause a ____. a. b. c. d. Hard brake pedal Soft (spongy) brake pedal Low brake pedal Slight hiss noise when the brake pedal is depressed Automotive Brake Systems, 5/e By James D. Halderman 73 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

CHAPTER QUIZ 7. An accumulator such as that used on electric or hydraulic brake boosters ____. a. b. c. d. Reduces brake pedal noise Provides higher force being fed back to the driver’s foot Provides a reserve in the event of a failure Works against engine vacuum Automotive Brake Systems, 5/e By James D. Halderman 74 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

CHAPTER QUIZ 8. The first step in diagnosing a Hydro-Boost problem is ____. a. b. c. d. A pressure test of the pump A volume test of the pump To tighten the power steering drive belt A thorough visual inspection Automotive Brake Systems, 5/e By James D. Halderman 75 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

CHAPTER QUIZ 9. A brake pedal feels spongy when depressed. Technician A says that a defective hydraulic brake booster could be the cause. Technician B says that a defective vacuum brake booster could be the cause. Which technician is correct? a. b. c. d. Technician A only Technician B only Both Technicians A and B Neither Technician A nor B Automotive Brake Systems, 5/e By James D. Halderman 76 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.

CHAPTER QUIZ 10. If the engine stops running, the Hydro-Boost will not be able to provide any power assist for the brakes. a. b. True False Automotive Brake Systems, 5/e By James D. Halderman 77 Copyright © 2010, 2008, 2004, 2000, 1995 Pearson Education, Inc. , Upper Saddle River, NJ 07458 • All rights reserved.