Hydraulic Brake Booster System Fundamentals and Service

Question 1

1. Name three (3) categories of power assist brake systems.

Answer 1

1. a) vacuum booster systems
   b) air-over-hydraulic booster systems
   c) hydraulic-over-hydraulic booster systems

Question 2

2. Explain why vacuum type power brake boosters are not generally used on vehicles with a diesel engine.

Answer 2

Diesel engines do not create sufficient vacuum to effectively operate vacuum boosters.

Question 3

needs question

Answer 3

e) both a and c must exist.

Question 4

Describe a quick test of a vacuum booster system to see if any boost exists at all.

Answer 4

With engine and key off, apply brakes several times to remove all vacuum. Apply brakes and hold foot down. Start engine and/or tum key on. Vacuum source is activated and brake pedal moves toward floorboard.

Question 5

Vacuum boosters work because a difference exists between a low-pressure area and a high-pressure area. a) true b) false

Answer 5

a) true

Question 6

Air-over-hydraulic booster systems are typically used when the vehicle has a gasoline engine. a) true b) false

Answer 6

b) false; Generally used with diesel engine and air compressor.

Question 7

Air used in the air-over-hydraulic booster system is: a) atmospheric air pressure. b) vacuum pressure. c) vented air pressure. d) compressed air.

Answer 7

d) compressed air.

Question 8

Water in the air supply system used with air-over-hydraulic booster systems does not have a negative effect on booster operation. a) true b) false

Answer 8

b) false; Water in air supply can affect valve operation/sealing.

Question 9

A power cluster is a combined air cylinder and hydraulic cylinder assembly. Explain how the air cylinder is activated.

Answer 9

The operator pushes a brake pedal, which activates an air control valve. Compressed air is directed from the air reservoir to the air cylinder on the power cluster.

Question 10

In the air-over-hydraulic brake booster system, a special hydraulic oil is used in the hydraulic system. a) true b) false

Answer 10

b) false; In the system , brake fluid is the hydraulic fluid used.

Question 11

Explain where brake fluid is used in the air-over-hydraulic brake system.

Answer 11

Brake fluid is used in the master cylinder and the lines leading to the wheel brakes.

Question 12

When bleeding the brakes in an air-over-hydraulic brake system, reduced air pressure is used in the air system. Explain why this is done.

Answer 12

A low air pressure is used so the brake fluid will not leave the bleeder screw at a high velocity. Eye protection is required.

Question 13

Hydraulic-over-hydraulic power brakes use fluid in two separate systems. This is described more accurately as: a) hydraulic fluid over hydraulic fluid. b) brake fluid over brake fluid. c) brake fluid over hydraulic fluid. d) hydraulic fluid over brake fluid.

Answer 13

d) hydraulic fluid over brake fluid.

Question 14

In the hydraulic-over-hydraulic system, pressure for the hydraulic system is normally supplied by a/an: a) hydraulic pump. b) power steering pump. c) electric/hydraulic pump. d) master cylinder.

Answer 14

b) power steering pump.

Question 15

What type of fluid might be used in the brake system of a hydraulic-over-hydraulic brake system?

Answer 15

DOT 3, DOT 4, DOT 5.1 or DOT 5 brake fluid. DOT 3, DOT 4 and DOT 5.1 can be mixed. DOT 5 is used alone.

Question 16

The hydraulic-over-hydraulic brake system uses high-pressure and low-pressure steel lines and flex hoses. Explain where high-pressure lines would be used.

Answer 16

High-pressure lines are used in the pressure supply side of the hydraulic system. High-pressure lines are used in the brake fluid lines.

Question 17

Explain the function of an accumulator as used in a hydraulic-over-hydraulic brake system.

Answer 17

The accumulator is used to provide high pressure to the power brake system if the power steering pump fails or the engine quits. This output is limited to two or three power-assisted pedal applications.

Question 18

For personnel working around accumulators several dangers may be present. Explain the dangers and how you would protect yourself.

Answer 18

High spring tension and high hydraulic fluid pressure exist in the accumulator. Relieve oil pressure before opening any line. Secure the spring while opening the accumulator. Follow manufacturer’s instructions.

Question 19

Explain the function of an electric/hydraulic pump as used in a hydraulic-overhydraulic brake system.

Answer 19

The EH pump supplies a lower pressure to the booster when the power steering pump is unable to maintain high pressure. If the power steering pump fails totally or the engine quits the EH pump takes over to provide partial power braking.

Question 20

Provide a basic hydraulic fluid flow path between the major system components in a hydraulic-over-hydraulic brake system.

Answer 20

Power steering pump to brake booster to steering gear to pump reservoir; or, power steering pump to steering gear to brake booster to pump reservoir.

Question 21

Contaminated hydraulic fluid in a hydraulic-over-hydraulic brake system only affects the brake booster. a) true b) false

Answer 21

b) false; Hydraulic fluid is also used in the power steering pump.

Question 22

If a visual inspection of the external components in the hydraulic system does not reveal any problems what would be the next step in the troubleshooting process?

Answer 22

Relieve system pressure. Install appropriate pressure test instruments. Use manufacturer’s test instructions. Compare readings to specifications. Relieve system pressure before removing test equipment.

Question 23

Explain how to relieve the hydraulic system pressure in a hydraulic-over-hydraulic brake system. What other safety precaution should be followed?

Answer 23

Apply and release the brakes several times. Wear eye protection. Crack a line to relieve residual pressure.

Question 24

Electrical and electronic components are becoming more common on brake systems. Describe some of the typical faults that can cause erratic or inaccurate electrical signals.

Answer 24

Loose, corroded and damaged electrical connectors, and broken, frayed and damaged conductors are the most common problem areas.

Question 25

A vehicle equipped with sensors, computer(s) and electronic modules requires extra care during troubleshooting. Describe the type of infonnation the manufacturer may provide to assist the technician.

Answer 25

Manufacturers provide diagnostic flow charts, wiring schematics, connector and pin descriptions and component location charts. A step-by-step troubleshooting sequence must be followed.

Question 26

Explain the precautions required when testing electrical and electronic circuits and components.

Answer 26

Do not short or ground electrical connections unless it is a part of a recommended test procedure. Use proper test equipment. Confirm electrical pin locations before probing with a test instrument.

Question 27

Valve bodies that have deep scores can be honed and reused. a) true b) false

Answer 27

b) false; Deep scores usually mean a new valve body assembly is required.