1. 4/15/2017
PORTER
AND
CHESTER
INSTITUTE

2. Brake Hydraulic Systems

3. Function of the Hydraulic System
The primary purpose of an hydraulic system is to transfer force from the brake pedal to the brake shoes and pads.
A hydraulic system can also be used to multiply force – in the same manner as a lever multiplies force.

4. Advantage of using hydraulics
Can transfer force over long distances
Can be easily routed around obstacles
Can transfer force to components that move with the suspension
Never needs adjustment

5. Most liquids are non-compressible.
Pascal’s Law
Most liquids are non-compressible.
When pressure is applied to a liquid in and enclosed system, that pressure is distributed equally and in all directions throughout the fluid

6. Pressure = Force ÷ Area
100 LBS
The amount of pressure is determined by the force applied to the piston, divided by the area of the piston
Piston Area =
1 Square Inch
100 ÷ 1 = 100 LBS Per Square Inch

7. Distribution of Pressure
Pressure is distributed equally and in all directions.
30 PSI
30 Pounds
30 PSI
1 Square
Inch
30 PSI
30 PSI
30 PSI

8. Cylinder Bore and Stroke
The size of the piston is generally expressed as its ‘bore’ diameter.
The distance that the piston moves is called the ‘stroke’.

9. The relationship between area and pressure
100 LBS
100 ÷ .5 = 200 LBS Per Square Inch
1/2 Square Inch
A smaller piston will generate more pressure

10. The relationship between area and pressure
100 LBS
100 ÷ 2 = 50 LBS Per Square Inch
2 Square Inches
A larger piston will generate less pressure

11. Using hydraulic pressure to transmit force
Master Cylinder
Slave Cylinder
Hydraulic Line

12. If Bores of Master and Slave Cylinders are the same
If the bore of the master cylinder is the same as the bore of the slave cylinder then the amount of force generated at the slave is the same as the force applied to the master cylinder.
The stroke of the slave cylinder will be the same as the same as the stroke of the master cylinder.

13. Master and Slave with Same Size Bores
Master Cylinder
Slave Cylinder
100 Pounds
100 Pounds
1 inch
1 inch
Hydraulic Line

14. Hydraulic Advantage
If the master cylinder is smaller than the slave cylinder there will be a multiplication of force
There will be more force output at the slave than input at the master
The stroke that the master cylinder piston moves through will be longer
The ratio of the area of the slave cylinder bore ÷ the area of the master cylinder bore determines the hydraulic advantage

15. If Master and Slave have Different Bore Diameters
Master Cylinder
Slave Cylinder
100 Pounds
400 Pounds
1 inch
¼ inch
Bore = 1 Sq. In.
Bore = 4 Sq. In.

16. Using Hydraulic Pressure to Transmit Force
Flexible rubber
hose
A hydraulic master and slave cylinder can transfer force over long distances without loss of power.
It can also transmit force through sharp angles and around obstacles easily
It can easily connect components that are vibrating or rocking - as in a clutch linkage.

17. Transferring force over a distance
A hydraulic master and slave cylinder can transfer force over long distances without loss of power.
It can also transmit force through sharp angles and around obstacles easily
It can easily connect components that are moving slightly as in a clutch linkage or suspension system.

18. Force Multiplication Using Leverage
1 square inch cylinder bore
50 Pounds at the brake pedal 2”
Pivot Pin 8”
Clevis Pin
200 Pounds at the master cylinder piston
200 PSI
200 PSI
A mechanical advantage of 4 to 1 is the result of the ratio of the length between the brake pedal and pivot point divided by the distance from the pivot point to the clevis pin

19. Force Multiplication Using Hydraulic Advantage
300 lbs.
800 lbs.
300 lbs.
Pivot Pin
Caliper bore = 4 square Inch
Wheel cylinder bore = 1.5 square Inch
200 PSI
200 PSI
300 lbs.
The amount of pressure at the brakes is equal to the size of the cylinder bores multiplied by the pressure
800 lbs.
300 lbs.

20. Brake Fluid

21. Properties of Brake Fluid
Non-Compressible
Very low freezing point
Very high boiling point
Non-Corrosive
Compatible with rubber used in brake system
Good lubricant
Hygroscopic [absorbs water]
Compatibility with other brands and types of brake fluid

22. DOT Sets Standards
The US Department of Transportation [DOT] sets specifications and standards for brake fluid
The SAE [Society of Automotive Engineers] also sets standards

23. DOT designation is based on boiling temperature
DOT 3 – Most Common Type in use today
DOT 4 – Higher Boiling Point than DOT 3
Often required by European car manufacturers
DOT 5 – Highest Boiling Point Available
Used for racing and super high performance vehicles

24. DOT Fluid Boiling Point
Dry Boiling Point
401
446
500
Wet Boiling Point
284
311
356
DOT 3
DOT 4
DOT 5

25. Why is Boiling Point Important?
When DOT 3 fluid becomes fully saturated with moisture its boiling point drops 1170 F
When brake fluid boils it forms gas bubbles
The gas bubbles are compressible – just like air
If the brake fluid is allowed to boil the pedal will go to the floor and the vehicle will not slow or stop!

26. Why Does Brake Fluid Get Saturated With Moisture
The flexible rubber hoses that connect the master cylinder to the calipers and drums allow a small amount of moisture into the system through Osmosis
Whenever the fluid filler cap is removed for service the fluid is exposed to the moisture in the air

27. POLY-GLYCOL BASED FLUID
Made of Glycol-Ether compounds
Made from vegetable oil not from petroleum
Very hygroscopic – absorbs moisture
Is used for both DOT 3 and DOT 4 fluids
Additives are used in both DOT 3 and DOT 4 fluid to neutralize water held in suspension
Will dissolve or discolor the paint on most cars!

28. LMA TYPE BRAKE FLUID ‘LOW MOISTURE ACTIVITY’
Recommended by many European car manufactures
Additives in fluid help prevent deterioration of rubber seals in hydraulic system made from natural rubber compounds
Manufactured by Castrol as a DOT 4 type fluid

29. SILICON BASED FLUID Very high boiling point
Marketed in the 1980’s as a DOT 5 fluid
Not hygroscopic – does not mix with water at all
Tends to aerate when cycled rapidly
This property makes silicone based fluid unacceptable for modern cars with ABS brake systems!
Has no harmful effect on painted surfaces

30. DOT 5.1 Fluid
Recently brake fluid manufactures have been able to formulate a poly-glycol based fluid that meets the requirements for DOT 5 fluids.
Since DOT 5 has been associated with silicone based fluid this non silicone fluid is referred to as DOT 5.1

31. What Type of Fluid Should Be Used
Most modern cars have the fluid type printed on the master cylinder filler cap

32. Handling Brake Fluid
Brake fluid must always be kept in a sealed container
If the cap is left off a container of brake fluid for 24 hours or more it should be discarded
Never use an open container of fluid that you don’t recognize or remember where it came from

33. Handling Brake Fluid
Brake fluid is clear or slightly amber in color – just like many other fluids – just because its in a container that says its brake fluid doesn’t necessarily mean it actually is brake fluid or is not contaminated with other fluids
The risk of injury to people and property is too great to justify saving a few cents

34. Handling Brake Fluid
All new containers of brake fluid have a seal under the cap that must be broken before use
If the seal is broken and you are not sure how old the fluid is – discard it !
Never transfer new brake fluid into another container

35. Handling Brake Fluid
Do not allow brake fluid to come into contact with the painted surfaces of the car body
If you accidentally get brake fluid on the paint work of a car – rinse it off with a garden hose – do not wipe it off with a rag
©2005 PORTER AND CHESTER INSTITUTE / CONNECTICUT SCHOOL OF ELECTRONICS

36. Handling Brake Fluid
Brake fluid should be changed every 2 years or 30 k miles
Brake fluid turns grey and then black as the rubber components of the brake system deteriorate
If you get brake fluid on you skin rinse it off immediately
If you get brake fluid in you eyes – thoroughly rinse your eyes with clean water – see a physician immediately!

37. Petroleum Based Fluids & Brakes
If a petroleum based fluid; motor oil, transmission fluid, power steering fluid – is accidentally introduced into the brake hydraulic system the rubber components will be destroyed.
All of the brake hydraulic components must be replaced – the steel lines can be flushed out with alcohol

38. Petroleum Based Fluids & Brakes
A sure sign of fluid contamination is distortion of the rubber seal on the master cylinder filler cap
If this seal appears to be melted, stretched, swollen or otherwise distorted the vehicle is unsafe to drive
A complete overhaul of the brake hydraulic system will be required

39. Master Cylinders

40. Single Piston Master Cylinder
Used prior to 1968
Vent Port
Replenishing Port
Fluid
Reservoir
Single Spool Type Piston
High Pressure Lip Seal
Low Pressure Lip Seal

41. Single Piston Master Cylinder
When Brake Pedal is Depressed
Piston is Moved Forward
by Brake Pedal
Lip Seal Closes
Vent Port
Pressure Rises in Chamber
Pressurized Brake Fluid is Sent to Wheel Cylinders

42. Master Cylinder Operation
When the lip seal passes over the vent port pressure builds up in the chamber

43. Pumping the Brake Pedal
When the brake pedal is cycle rapidly fluid can be drawn into the working chamber through holes drilled in the piston land and around the lip seal

44. Single Piston Hydraulic System
One piston / cylinder provides all the fluid pressure for all 4 brakes

45. Single Piston Hydraulic System
When the brake pedal is depressed fluid is distributed evenly to all 4 wheel cylinders

46. Brake Failure in a Single Piston System
If any component in this system fails the entire brake system is inoperative

47. Dual Piston Master Cylinder
All Modern Cars and Trucks built after 1968 use a Dual Circuit Master Cylinder

48. Master Cylinder Components
Reservoir
Primary Piston
Cylinder Body
Secondary Piston
Return Springs

49. Dual Hydraulic Circuits
Having two separate hydraulic circuits insures that we will have at least two working brakes in the event of a failure

50. Dual Hydraulic Circuits
Having two separate hydraulic circuits insures that we will have at least two working brakes in the event of a failure

51. Dual Hydraulic Circuits
If one of the rear wheel cylinders has a leak there will still be adequate hydraulic pressure to stop the car with the front brakes

52. Master Cylinder Operation
Primary
Chamber
Secondary
Chamber
In normal braking pressure developed in the primary chamber is applied to the secondary piston
The pressure on the rear of the secondary piston is transferred into the secondary chamber

53. Primary Circuit Failure
Extension on the primary piston makes contact with secondary piston
If the primary circuit fails [leaks] the primary piston will until forward it makes contact with the secondary piston

54. Secondary Circuit Failure
If the secondary circuit fails the secondary piston will bottom out in the cylinder bore
The primary piston will still be able to develop pressure

55. Vent Ports
Vent Ports {Compensating Ports} allow fluid to flow into the chamber when the brake pedal is not depressed
Fluid returns to the reservoir through the vent port when the brakes are released

56. Replenishing Ports
The replenishing port allows fluid from the reservoir to fill the low pressure chamber behind the front piston land
The replenishing port is always open to the reservoir

57. Quick Take Up Type Master Cylinder
Provides better fuel economy by reducing brake drag

58. Quick Take Up Type Master Cylinder
Introduced in 1980 on GM X-body
Adopted by many manufactures since
Used to reduce rolling drag caused by contact between pad and rotor
Special design caliper pulls brake pad slightly away from rotor when brakes are not applied
Additional fluid volume is needed to compensate for the fluid that is displaced by pad pull back

59. Small forward bore for high pressure in normal braking
Step Bore Design
Larger rear bore – for additional fluid displacement of quick take up calipers
Small forward bore for high pressure in normal braking

60. 2 Stage Operation Initial ‘take up’ stage
Extra volume of rear section of cylinder is pushed over and around lip seals and enters both operating chambers
‘Take up volume’ pushes pads out into contact with rotor
Until pads come into contact with rotor pressure in system is very low

61. Take-Up Stage
When the brakes are initially applied the extra volume of fluid in the rear chamber passes around the lip seals of the primary piston and into the primary chamber

62. Pressure Stage Once pads contact rotor pressure rapidly builds up
Fluid pressure in large rear section of master cylinder is allowed to vent back to reservoir through ‘quick take up valve

63. Quick Take-up Valve opens
Take-Up Stage
When the pads come into contact with the rotors pressure builds up
The quick take up valve opens to allow fluid in the rear chamber to vent to the reservoir
Quick Take-up Valve opens

64. Brake Circuits

65. Weight Applied to Tires
2250 lbs.
1250 lbs.
The brake force needed to bring the vehicle to a stop is proportional to the weight applied to each set of wheels [axle]

66. How Weight Effects Brake Performance
The more weight that is applied to a wheel - the harder the brake must work to slow that wheel
Nearly all vehicles have the engine and transmission located over the front axle
In general the brakes at the front axle must work harder than those at the rear because they carry more of the vehicle weight

67. Weight Transfer During Braking
Decreased weight on rear wheels
Increased weight on front wheels
Weight
All vehicles experience a weight transfer toward the front of a vehicle as the vehicle is brought to a stop

68. Weight Transfer
Weight transfer during deceleration puts an additional load on the front brakes.
The inertia of the vehicle during braking transfers force from the rear to the front effectively decreasing the weight on the rear wheels and increasing the weight applied to the front wheels

69. Front Brake Bias
Because of the extra weight the front wheels carry and the weight transfer to the front during braking the front brakes do more of the braking than the rear
For this reason the front brakes must be larger and more effective than the rear brakes
Trucks that haul heavy loads are the only type of vehicle that require rear brakes that are equal to or more effective than the front brakes

70. Brake Bias
Brake Bias is the term given to the relative percentage of braking work done by the front and rear brakes of a vehicle in a forward stop.
Brake bias is expressed in percentage on the front and rear axle [ 50/50, 60/40 etc.]

71. Typical Brake Bias Percentages
RWD 50/50
FWD 70/30
Truck – varies with load

72. 50 / 50 Brake Bias on RWD
50 of braking done by front brakes
50 of braking done by rear brakes
Rear wheel drive vehicles have a brake bias the ranges from 50/50 to 60/40

73. Brake Bias for Front Drive
70% of Brake Effort done by the Front Brakes
30% of Brake Effort done by the Rear Brakes
Front wheel drive cars have a front rear brake bias of 70/30 and higher

74. Hydraulic Circuits
Because there is fundamental differences in the requirements for a rear drive brake system and a front drive brake system, two different types of hydraulic system are needed
Rear drive vehicles use a front / rear split hydraulic system
Front drive vehicles use a dual diagonal hydraulic system
In addition to the type of hydraulic circuit used in front drive and rear drive vehicles there are difference in the types of hydraulic control valves as well

75. Front /Rear Split Hydraulic Circuit
Used on nearly all Rear Drive cars and light trucks

76. Front / Rear Split Dual Circuit
One chamber of the master cylinder feeds both front brake cylinders – the other chamber feeds the rear cylinders
As the brake bias is nearly 50/50 if one circuit fails due to a leak the other circuit can provide nearly 50% of the normal braking effort
This will allow the vehicle to stop within a reasonable distance [although not as quickly as with a fully functioning braking system]

77. Front / Rear Split Dual Circuit
On a disc / drum system one chamber of the master cylinder will feed only the disc brakes
Drum Brakes
Disc Brakes
Since the disc brakes require a greater volume of fluid the fluid reservoirs will have different volumes – the large reservoir will be for the front brakes the smaller one for the rear

78. Why a Front/Rear Split Wont work on FWD
If a front / rear split were used on a FWD vehicle; then if a failure occurs in the front hydraulic circuit, the rear circuit could provide only 30% of the braking effort needed to slow the vehicle
Since there is so little weight carried by the rear wheels increasing the rear braking efficiency would cause the rear wheels to lock up – thus promoting a skid

79. Why a Left / Right Split Wont Work
If the brake system is split left / right then in the event of a failure only the brakes on one side of the vehicle would work
In a panic stop the vehicle would swerve left into oncoming traffic or right into the gutter

80. Dual Diagonal Brake Circuit
Used on all FWD vehicles

81. Dual Diagonal Brake Circuit
The solution for front drive brake hydraulic circuit was to split the system on a diagonal
Connecting one chamber of the master cylinder to the left front and right rear brakes
The other chamber is connected to the right front and left rear

82. Dual Diagonal Brake Circuit
In the event that any one hydraulic component fails there will be one front and one rear brakes still working
The two working brakes will be on opposite sides of the car [Left to Right] so there will be no pull if one circuit fails
©2005 Porter and Chester Institute / Connecticut School of Electronics

83. Trucks and SUVs
Trucks and SUVs have a significant portion of their weight located over the rear axle and will normally use front / rear split brake systems
Light unit body SUVs based on FWD drivelines [Subaru, Honda CRV etc.] use dual diagonal brake systems as they have a high forward brake bias

84. PORTER
AND
CHESTER
INSTITUTE