Chapter 4

Hydraulic System

Fundamentals

After studying this chapter, you will be able to: o Explain the use of hydraulic pressure in braking systems. o Explain the incompressib ili ty of liquids as the basis of hydrau lic system operation. o Exp lain how movement is transferred by way of liquid s. o Explain how cylinder size changes hydrau lic force and distance traveled.

Explain Pascal 's law.

o Identify the components of a simple brake hydrau li c system. o Identify the qualities of brake fluid and fluid c lass ification s. o Explain the importance of using the proper b,ake fluid.

Important Terms

Hydrauli cs Master cy l i nder Ca lipers Department of Transportation (DOT) Cylinders Spl it brake system Control va Ives Society of Automotive Engineers (SAE) Pistons Wh eel cylinders Hydrauli c actuator Hygroscopi c

Pascal's law

61

62 Auto Brakes

You learned in Chapter 1 that the earliest brake systems were mechanical devices. You also learned that modern brake systems are operated by hydraulic pressure. Using hydraulics to apply the brakes is more efficient and convenient than mechanical linkages. The hydraulic system is designed to allow brake pedal pressure to be increased many times for greater braking power and to distribute braking force equally. This chapter will explain in detail the use of hydraulic pl'essure in braking systems.

The Basics of Hydraulic Systems The study of liquids and how they work is called

hydraulics. You wi II not requ i re an extensive education in hydraulic theory, but you must know how hydraulic pressure is created and used. The information in this chapter is a brief overview of how hydraulic pressure is put to work in the modern brake system. If you understand what is presented here, you will be able to perform brake hydraulic system service.

Incompressibility of Liquids Gases such as air can be compressed, that is, made to

take up less space. Compressed air is used in the shop to power air tools and other equipment. However, liquids such as water, oil , and brake fluid cannot be compressed. The inability of liquids to compress is the basis of brake hydrau I ic system operation, as well as the operation of power steeri ng and automatic transm iss ions.

Figure 4-1 compares the compressibi I ity of gases and liquids. A weight is placed on the lids of closed containers of gas and liquid . When weight is applied to a container of gas, its volume is reduced. In other words, a gas under pressure takes up less space than it does when not under pressure. Placing the weight on the liquid increases its pressure, but does not reduce its volume.

Transfer of Movement Since liquids are not compressible, they can be used

to transfer movement. Figure 4-2 shows a simple hydraulic system consisting of two cylinders and two pistons, connected by tubing called a hydraulic line. Both the right and left cylinders and pistons are the same size. The entire system is filled with liqUid. If the piston on the left side is pushed, it pressurizes the liquid. This pressure on the liquid is transmitted through the hydraulic line to the other piston, causing it to move.

It is important to note that moving the first piston caused the second piston to move exactly the same amount. This is because the force was transmitted through the liquid unchanged. As long as the input and output piston areas are the same, input force is transmitted unaltered. The cylinder and piston assembly shown in Figure 4-2 is the basis of all hydraulic brake system design and operation.

Changes in Force There are times when it is desirable to change the

amount of movement of the input and output pistons. In a simple hydraulic system, changing the cylinder size changes both the distance traveled and the amount of force applied. In Figure 4-3, note that the input cylinder has an area of one square inch and the output cylinder is 10 in2. If we apply a force of 1 lb. to the input cylinder, this produces a pressure of 1 pound per square inch, or 1 psi in the system. This pressure travels to the output piston. At the output piston, the 1 psi acting on the 10 square inch piston produces an output force of 10 pounds. Note that by varying the size of the input and output pistons, we have increased the input force 10 times.

501bs. I

A B

No pressure 50 Ibs. pressure

2

Liquid

A B

Figure 4-1. 1-Air (gas) is compressible. A-There is no pressure on the piston. B-Pressure has forced the piston down, compressing the air trapped in the container. 2-Liquids cannot be compressed. When pressure is applied to the piston in B, it does not compress the liquid.

HYdrauliC line

/ ,----.,""""--""7\

' CYlinder

Figure 4-2. Transfer of movement by using a liquid and hydraulic line (tubing).

63 Chapter 4 Hydraulic System fundamentals

Cylinder \ ~----",~---:"

x1 distance

x10 force

Figure 4-3. Changing a cylinder's size will alter the piston travel and force.

However, this increase in hydraulic force has a price. The input piston moved one inch while the output piston moved 1/10 of an inch. Therefore, the force was increased ten times, but the amount of movement was decreased to 1/10. Therefore, to increase force we must accept a decrease in movement. How this principle is put to work in the modern brake system wi II be covered later.

Master "", t I; epedalcylinder '"-- ~l ,~:q:;:;;:::X\

Force

pistons and Rotor~ linksL-'~:J '

Front

pressure

Brake lines (tubing) t

Wheel cylinders ~ . _ -

v -,-.----, ' iZ2L7_LZ

Figure 4-4. Creating pressure in the master cylinder, causes pressure to build in the entire system. (Chevrolet)

Pascal's law The simple brake system shown in Figure 4-4 illus

trates a basic law of hydraulics, called Pascal 's law. Pascal's law states the pressure in a closed hydraulic system is the same everywhere in the system. This principle of hydraulics was discovered in 1653 by Blaise Pascal.

When the master cylinder at the top of Figure 4-4 is pressurized by pushing on the piston, the pressure created is the same throughout the system. This principle allows you to check pressure at one point in the system with confidence that the pressure is the same elsewhere in the system. It also allows you to check variations in pressure and determine whethel' they are normal or caused by system defects.

A Simple Brake Hydraulic System The following illustrations show the design of a sim

ple on-vehicle brake hydraulic system having front disc and rear drum brakes. Follow the illustrations and text to see how this simple system makes use of the principles of hydraul ics.

Master Cylinder In Figure 4-5, the master cylinder provides the pres

sure to operate the other hydraulic components. The master cylinder is in turn, operated by the foot pedal. Pushing on the pedal creates a force on the pistons, creating hydraulic pressure.

Note the master cylinder has two pistons. The purpose of this design is to split the brake hydraulic system into two parts. This design is used on all modern vehicles, and is called a split brake system. If one side of the system loses hydraulic fluid due to a leak, the other side will still function. In operation, pushing on the first piston causes the buildup of pressure in the first chamber. This, in turn , pushes on the second pi ston, causing it to pressurize the second chamber. In both systems, the pressure is sent to the wheel brake units. Split brake systems are discussed in more detail in Chapters 5 and 6.

Wheel Cylinders and Calipers The wheel cylinders and calipers are the output

devices of the brake hydraulic system. In Figure 4-5, the disc brake caliper pistons are much larger than the wheel cylinder pistons. Thi s allows the front brakes to be applied with more force. The reason for this is that inertia places more of the vehicle's weight on the front brakes during stopping, allowing the rear brakes to brake for control. These factors will be discussed in more detail in Chapter 11.

Also note the front caliper pistons are much larger than the master cylinder piston. This means the caliper pistons will multiply the brake pedal force many times. However, the brake pedal must be pushed a long distance

64

Dual piston ~ master cylinder

Pistons

Caliper

Hydraulic lines

cylinder

Figure 4-5. The master cylinder generates the pressure needed to operate the calipers, wheel cylinders, valves, etc.

to move the caliper piston a small amount. To keep the pedal tt"avel from being excessive, the caliper assembly is designed so the pads and rotor are very close together when the brake is not applied. Therefore, less than .5" (12.7 mm) movement of the brake pedal will apply the brakes. Slightly misadjusted or worn brakes will greatly increase brake pedal travel.

Lines and Hoses The various components of the brake hydrau I ic system

are connected through lines and hoses. Lines are made of steel and hoses are made of braided rubber. Lines connect the stationary parts of the hydraulic system and hoses connect the parts which move in relation to each other.

Control Valves The modern brake system contains several flow and

pressure control valves. These valves are: D Metering valve. D Proportioning valve. D Residual pressure valve. D Pressure differential valve.

The job of these valves is to make the brake system more efficient, and to warn the driver when a failure occurs. They perform these jobs by using spring pressure to oppose hydraulic pressure. A brief explanation of these valves is given in the sections that follow. Note that two or more of these valves are sometimes installed into a single assembly called a combination valve.

Metering Valve The metering valve is used to keep the front brakes

from applying before the rear brakes. The front brake pads are not held in the retracted position by springs as

Auto Brakes

are the rear brake shoes . Therefore, if the same amount of pressure reached the front and rear brakes at the same time, the front brakes would do all the stopping and wear prematurely. Applying only the front brakes could also cause instability, or might throw the vehicle into a skid.

The metering valve assembly, Figure 4-6, consists of a small spring-loaded valve. The spring seats the valve against hydraulic pressure until a certain pressure is reached. At this pressure the valve is unseated and fluid can flow to the disc brake calipers. This delay in applying the front brakes gives the rear brakes time to overcome spring pressure and begin applying the rear brakes.

Proportioning Valve If the brakes are applied hard during an emergency

stop, much of the veh icle's weight is transferred to the front wheels. Hard braking can cause the wheels to lock up and skid. This is not only hard on the tires, it can cause dangerous instability, sometimes causing the vehicle to spin out of control.

To prevent this, a proportioning valve is installed in the rear brake line. A calibrated spring holds the proportioning valve open against brake pressure. figure 4-7 illustrates the proportioning valve. Under normal braking, brake fluid can flow to the rear brakes. Under a hard stop, however, brake pressure will exceed the calibrated tension of the spring, and increased pressure will close the valve against the opening. This limits brake pressure at the rear wheels to what is already in the rear hydraulic system. Preventing the development of additional pressure helps to prevent wheel lockup.

Metering valve

Figure 4-6. A metering valve is added to the system to prevent the front brakes from applying before the rear brakes are actuated.

65 Chapter 4 Hydraulic System Fundamentals

Proportioning valve1'-' / 0 .1

~ Jlb Figure 4-7. The proportioning valve helps to limit brake pressure to the rear brake system, reducing the chance of wheel lockup.

Note: On many modern vehicles, proportioning valves are used in combination with disc brakes. Proportioning valve

usage will be covered in more detail in Chapter 9.

Residual Pressure Valve If the vehicle has drum brakes, a small amount of pres

sure must be maintained in the system to keep the wheel cylinder lip seals from collapsing. The residual pressure valve is used on vehicles with drum brakes to maintain this pressure. Figure 4-8 illustrates this valve design. Note that the actual valve assembly is a one piece unit.

In operation, the valve to the right is unseated and fluid flows to the rear wheels. When the brake pedal is released, the valve to the right closes, and fluid returns to the master cylinder through the valve to the left, which opens against spring pressure. When the fluid pressure becomes low enough (usually about 7-10 psi or 48.26-68.95 kPa), it no longer has enough force to keep the spring compressed. The spring then closes the valve, trapping a small amount of p"essure in the rear brake system.

Pressure Differential Valve The pressure differential valve is a warning device

used in all split brake systems. The p"essure differential valve is a double-sided valve. The valve is installed so that each line presses on one side of the valve. A typical pressure differential valve is shown in Figure 4-9. . When both sides of the hydraulic system are operatiIlg normally, the valve is centered and the switch has no way to co~plete the circuit. When one side of the system fails, pressillg ~n the brake pedal will result in normal pressure on one Side of the system, and lower than normal

pressure valve assembly

r--r.:".,--------r.''7rl

Figure 4-8. The residual pressure valve helps to maintain a specific brake fluid pressure in the drum brake system.

valve assembly ,....,...,-r-- ----r.o-r-.

Figure 4-9. Split brake system which incorporates a pressure differential valve unit.

pressure on the other side. This causes the valve to move to the side with less pressure. When the valve moves, the switch is grounded. Electrical current flows through the switch and illuminates a dashboard light.

Basic Anti-lock Brake Hydraulics

. Detailed operation of the anti-lock brake system (ABS)

will be covered in detail in Chapters 21 and 22. However the anti-lock brake hydraulic system should be mentioned

66 Auto Brakes

since it has many sim il arities and a few differences w ith the standard brake system . The major difference between the anti-lock and standard brake systems is the anti -lock brake hydrau I ic actuator.

The anti-lock brake hydraulic actuator conta ins control valves, similar to those di scussed earlier. These va lves are operated by electric solenoids or small motors . One type of anti-lock brake hydraulic actuator is shown in Figure 4-10. The va lves ca n seal off parts of the brake hydrau lic system, in a manner simil ar to the proportioning va lve discussed ear li er. The va lves can also dump fluid back to the master cyl inder, reducing system pressure. The va lve so lenoids are control led by electrical signa ls from an on -board computer.

Some anti-lock brake systems contain a pu mp, driven by an electric motor to develop ex tra pressure. This pump is usually a vane pump, Figure 4-11. The vane pump consists of a set of movable vanes attached to a slotted drum, usuall y cal led a rotor. The rotor and vanes are instal led inside an eccentric (egg-shaped) housing.

As the rotor turns, the vanes move in and out w ithin the slots. Pressure behind the vanes keeps them in contact with the eccentri c housing wa lls. As the rotor is turned by the electric motor, the vanes crea te a chamber which increases in size. This increase in size crea tes a suction at the intake port, drawing in fluid . As the vanes rotate through the eccentric housing, they reduce the size of the

chamber, creating pressure wh ich is discharged through the pump out let port. This pressure is then used in the antilock brake system.

Brake Fluid

Brake fluid is essential to the proper operation of the

brake system. In the last 60 years, many types of brake fluid have been tried. Modern brake fluid is manufactured accord ing to requirements of the Department of Transportation, usually cal led DOT, and the Society of Automotive Engineers, known as SAE. Other industry and government organizations also have standards for brake fluids.

Most brake fluids are derived from a mixture of nonpetroleum fluids, such as polyglyco ls, glycoethers, and other add itives which increase the fluid 's re liability. Brake fluid is also used in manual transmission clutch or slave cy linders.

Brake Fluid Standards Low quality brake fl uid w ill work in a brake hydrauli c

system, as wil l water, alcohol , or many other fluids - fOI' a short time. However, for maximum brake system life and the safety of the veh ic le occupants, brake fluid must conform to high standards.

5

A B

Figure 4-10. A-Actuator assembly during normal braking operation. 1-Applied master cylinder pressure. 2-Bypass brake fluid. 3-Normally open solenoid valve. 4-EMB braking action. 5-oc motor pack. 6-ESB braking action. 7-Gear assembly 8-8al/ screw 9-Check valve unseated. B-Actuator position during the anti-lock brake phase. 1-Trapped bypass brake fluid. 2-Solenoid valve activated. 3-EMB action released. 4-oc motor pack. 5-ES8 braking action released. 6-Gear assembly 7-Ball screw 8-Check valve seated. 9-Applied master cylinder pressure. (Delco Moraine)

67 Chapter 4 Hydraulic System Fundamentals

Reservoir ---------"'Q cap

Reservoir retainer

Valve bl ock assembly

Reservoir grommet

Pressure switch

Pump and motor assembly

T Pushrod \Brake fluid rear

reservoir

<

Push rod (front)

Spring

Pushrod assembly

High-pressure hose

Pump insulator

Return hose

Figure 4-11. Another type of anti-lock brake hydraulic actuator. (FMC)

Caution: Never add petroleum-based fluids to the brake system. Any type of motor oil , transmission fluid, or other petroleum

product will swell and destroy the rubber seals in the system.

Resistance to Boiling Th e most important req uirement of b,"ake fluid is its

res istance to boiling. If the brake fluid boils, it becomes a gas. As we learned ea rli er in thi s chapter, gases are compressib le. Therefore, if the brake f luid boils, pressing on the brake pedal w ill simply compress gas instead of appl ying the brakes. Since fri ctional heat is transmitted to the hydraulic system during braking, fluid resi stance to boiling is very important.

Water Absorption Another important quality of brake fluid is its ability to

absorb w ater. Brake fluids are intentionally desi gned to be hygroscopic, or able to absorb water. This woul d seem to

be a bad quali ty, since water will lower th e fluid 's resistance to boiling. Should water get into the brake system, however, it would tend to collect at low spots in the system. Water col lecti ng at one spot in the brake system could cause corros ion at that spot. Pure water would also freeze in co ld weather, blocking off brake lines or sti ck ing pi stons. Designing brake fluid to absorb water helps to minimize these problems.

You must take precautions to lim it the exposure of brake fluid to water. Brake fluid th at has a bo iling point of 446F (230C) when completely water free has a boiling point of 311 F (155C) when it has abso rbed the maximum amount of w ater it ca n hold, Figure 4-12A. Always keep containers tight ly capped, and do not leave hydraulic system parts disconnected for long periods of time.

Note: When servicing anti-lock and traction control systems, only use brake fluid from a newly opened container. This will

minimi:ze the chance of dirt and water entering the system.

68

280 536 \260 500As the water content

of brake fluid increases 464 over time, the boiling

240 () III220 4280 pOint decreases 0 C \'0 200 392 ~ 0.. "0 OJ 356 Q.c 180 .~ '0

CD 160 320 0 "T1 ~ot4

140 284DO~--120 248

100 212 0 2 3 4 5 6 7 8

% H20, wt.\wt. A

B

Figure 4-12. A-Brake fluid boiling points decrease as they absorb more moisture. 8-Tools are available for checking the moisture content of brake fluid. (Leica)

There are several products on the market to check brake fluid for water contamination. One type is shown in Figure 4-12B. Another type uses chem ically treated strips, which react in the presence of water or other fluids .

Brake fluid can also be checked for water contamination with an electronic brake fluid tester, Figure 4-13. To use this type of fluid tester, remove the cap on the brake fluid reservoir and insert the tester's probe into the fluid . The tester heats a small sample of the fluid and measures the temperature at which it boils. The tester then uses the boiling point to calculate the percentage of water in the fluid, which it displays onscreen.

Other Brake Fluid Requirements In addition, brake fluid must have other qualities. It

must lubricate movi ng parts of the brake system, such as the pistons and seals. The fluid must help to prevent corrosion of the metal parts as much as possible. Good brake fluid should not damage rubber seals or any other part of the brake system, and must flow easily, even at very low temperatures.

Auto Brakes

Types of Brake Fluids Brake fluid is classed by its DOT number. The two

common types of modern brake fluid are glycol based and silicone based. Glycol-based fluid is able to absorb water, and has a boiling pOint of over 400F (200C). Modern glycol based brake fluid is usually classified as DOT 3 or DOT 4. Both types are clear fluids, with a slight amber tinge. DOT 5 brake fluid can be easily spotted since it is purple in color.

The difference between DOT 3 and DOT 4 is their viscosity and ability to resist heat. DOT 4 can absorb more heat than DOT 3, but has a sl ightly lower viscosity than DOT 3. Silicone-based brake fluid is classed as DOT 5, and has a boiling point of over 500F (260C). Silicone brake fluid will not absorb any significant amount of water. For this reason, it is vital that no water or airborne moisture be allowed to enter a silicone fluid brake system or fluid conta i ners.

~ )3:,11 Caution: Glycol and silicone brake fluidsf5// are not compatible, and should never be o mixed. Do not use silicone brake fl uid in a vehicle equipped with an ASS system.

Federal and SAf Brake Fluid Standards In addition to DOT classifications, there are other

standards for brake fluid . These include Federal Motor Vehicle Safety Standard 116, or FMVSS 116, and Federal specification VV-B-680. The Society of Automotive Engineers (SAE) publ ishes specification jl703 . Look for these specifications when purchasing brake fluid . DOT 5.1 is another fluid type, used on a few vehicles. DOT 5.1 is not a silicone-based fluid, and should not be added to a brake system using DOT 5 fluid.

Figure 4-13. This tester boils a small sample of brake fluid to determine how much water is present. The screen displays the amount of water as a percentage. (OTC)

69 Chapter 4 Hydraulic System Fundamentals

Summary The brake hydraulic system has evolved over many

years and relies on the basic principles of liquids. The study of liquids and how they work is called hydraulics.

liquids cannot be compressed . This property of liquids is used to cause liquids to transfer movement and pressure, as well as increase force. The operation of the brake hydraulic system is based on Pascal's law which states that pressure is transmitted unchanged through a closed hydraulic system. The basic components of a brake system are the master cylinder, wheel cylinders and calipers, connecting lines and hoses, and various control valves.

The anti-lock brake system is operated by the same hydraulic principles as the standard brake system. ABS hydraulic control valves are operated by electric solenoids controlled by an on-board computer. Pressure is often supplied by a motor driven hydraulic pump.

Some of the most important properties of brake fluid are its ability to resist boiling, its ability to absorb water, and its resistance to corrosion. Brake fluid must also lubricate moving parts and stay liquid at very low temperatures.

Brake fluid is classified and rated by the u.s. Department of Transportation (DOT) and the Society of Automotive Engineers (SAE). The two major classes of brake fluid are glycol based and silicone based. The two types should not be mixed. Brake fluid containers should be kept closed to reduce the amount of water absorption.

Review Questions-Chapter 4 Please do not write in this text. Write your answers on

a separate sheet of paper. 1. A basic principle of hydraulics is that ___ cannot

be compressed. Air and other ___ can be compressed.

2. Liquids can be used to ___ motion and pressure.

3. Tubing that connects two hydraulic units is usually called a hydraulic ___

4. Changing the sizes of the input and output cylinders can change the amount of ___ applied.

5. State Pascal 's Law.

6. What are the two brake hydraulic system output devices?

7. The metering valve keeps the _ __ brakes from applying before the __ brakes.

8. To keep the rear wheels from skidding during a panic stop, a _ __ valve is installed in the rear brake line.

9. A residual pressure valve is used only on vehicles with brakes.

10. ABS systems usually use a __ pump to develop extra hydrau I ic pressure.

11 . Standards for brake fluid are set by two organizations. Name them.

12. Brake fluid is designed to absorb ___.

13. It is vely important that brake fluid resist _ __ at high temperatures.

14. Brake fluids classified as DOT 3 or DOT 4 are based. Brake fluids classed as DOT 5 are based. This fluid can be easily spotted since it is colored

15. Which of the above fluids should not be used in an ABS system?

1. Which of the following cannot be compressed? (A) Brake fluid . (B) Water. (C) Air. (D) Both A & B.

2. Technician A says that changing the sizes of the input and output pistons can change the distance traveled. Technician B says that changing the sizes of the input and output pistons can change the amount of force from the output piston. Who is right? (A) A only. (B) B only. (C) Both A & B. (D) Neither A nor B.

3. According to Pascal's law, the pressure in a closed hydraulic system is the same ___. (A) only at the input piston (B) on Iy at the output pistons (C) only at the input and output pistons (D) everywhere in the system

4. Technician A says that that the purpose of having two pistons in the master cylinder is to split the brake hydraulic system into two parts. Technician B says that the purpose of having two pistons in the master cylinder is to increase braking force. Who is right? (A) Aonly. (B) B only. (C) Both A & B. (D) Neither A nor B.

70

5. wheel cylinders devices of any brake

(8) output

fluid transfer

friction

6. Why are the front larger than the master cylinder piston?

To increase travel. To decrease pedal travel. To increase brake force. To decrease brake force.

7. All of the following statements about metering valves are true, EXCEPT:

the metering valve is used to keep the rear brakes from the brakes.

(B) if the valve was not used the veh ide be more prone to skid.

va overuse of the front

the metering valve the front brakes from out too soon.

valve which of the fol

Increases pressure to the rear brakes. Prevents pressure increase in the rear Increases pressure at the front brakes. Bypasses fluid to the master cylinder.

8. During hard

9. A says that the I pressure valve is on brakes. Technician B says

that the I'esidual pressure valve the cups col Who is right?

(A) A only. (8) B only.

Both A & B.

(D) Neither A nor B.

Auto Brakes

10. the the pressure

Restore equal pressure to each side of the system.

(8) are Illuminate a

(D) B & C 11. The pump in the typical ABS system is known as

a pump. (A) rotor

gear

(C) vane (D) piston

12. ABS valves are operated pump hydrau Iic pressure signals from a computer brake pedal pressure

from the cal and wheel cyl

13. The most of brake is its resistance to

boil

(B) freezing

corrosion

14. brake flu id to water lowers its

15. Silicone brake fluid carries what DOT number? 3. 4.

(C) 5.

Varies with the fluid