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1. Chapter Nine
Brake Hydraulic Theory

2. Objectives
 Discuss application of Pascal’s Law in
brake systems
 Explain purpose and operation of the
brake system
 Discuss use of power boosters
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3. Objectives
 Explain purpose and operation of master
cylinders
 Discuss and explain use of control valves
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4. Introduction
 Newton’s Law states that an object in
motion tends to stay in motion
 Brake systems supply equipment to slow a
vehicle and allow safe operation
 Brake system operation is based on
hydraulic theory
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5. Hydraulic Theory and Pascal’s
Law
 Hydraulic Theory Review
 Liquids cannot be compressed
 Force applied at one end of a sealed system
results in equal force at other end
 Fluid pressure can also control forces at work
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6. Hydraulic Theory and Pascal’s
Law
 Pascal’s Law
 Pressure applied to fluid in a sealed system is
transmitted equally in all directions and to all
parts of the system
 Force (F) = Pressure (P) x Area (A)
 Formula can also be used to calculate mechanical
advantage
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8. Brake System Operation
 Automotive Brake Systems
 Use hydraulic theory to transform driver’s input
(mechanical) to a output force (hydraulic) that
applies the brakes
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9. Brake System Operation
 Components of a basic brake system
 Pedal
 Linkage
 Master cylinder
 Lines and hoses
 Control valves
 Disc or drum brakes
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11. Brake System Operation
 Pedals and Linkages
 Mounting for pedal is a shaft from
which the pedal hangs downward
 Pedal is connected to master cylinder with a
pushrod
 Pushrod between pedal and booster are
usually adjustable
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12. Brake System Operation
 Pedals and Linkages
 Pushrod between booster and master
cylinder may be adjustable
 Mounting also holds brake light switch
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13. Brake System Operation
 Braking-as brakes are being applied:
 Driver depresses brake, brake fluid is
pressurized by master cylinder
 Forces fluid through lines to calipers or
wheel cylinders
 Fluid pressure is equally transmitted to each
component in brake system
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14. Brake System Operation
 Brake Fluids
 Transforms mechanical input of driver
applying pedal
 Brake fluids have different classifications and
must be used accordingly
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15. Brake System Operation
 Brake Fluids
 Fluid is hygroscopic (absorbs moisture from
air) and requires periodic service to maintain
boiling point safety
 Follow manufacturer’s recommendations,
usually 3yrs/36k miles
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17. Brake System Operation
 Power Boosters and Master Cylinders
 Master cylinder is transformation point
between mechanical and hydraulic
 Booster uses engine vacuum or hydraulic
pressure from power-steering to boost
pressure created by applying the brake pedal
 Combination of master cylinder and booster
creates additional pressure required to stop a
vehicle
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18. Brake System Apply Components
and Operation
 Master Cylinders
 Original systems were single-piston
 One failure disabled whole system
 Dual or split brake systems were mandated in
1967 for safety reasons
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19. Brake System Apply Components
and Operation
 Master Cylinders
 Dual or split systems can be front/rear or
diagonally split
 Dual systems can have a failure in one circuit
and still operate
 At reduced effectiveness
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20. Control Valves, Lines and Hoses,
and Warning Lights
 Metering Valves
 Vehicles with front discs and rear drums
 Must have metering valves for brake stability
 Controls application to manage braking
 Ensures rear brakes apply before front
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21. Control Valves, Lines and Hoses, and
Warning Lights
 Proportioning Valves
 Necessary to apply leading brakes more than
trailing brakes
 Control pressure to prevent rear-wheel
lockup during hard braking
 Combination valves combine proportioning
valves and metering valves
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22. Control Valves, Lines and Hoses, and
Warning Lights
 Load-Sensing Proportioning Valves
 Help prevent rear-brake lockup on trucks by
monitoring vehicle load
 One type monitors height between frame and
rear axle
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23. Control Valves, Lines and Hoses, and
Warning Lights
 Load-Sensing Proportioning Valves
 Another type is part of combination valve and
“senses” as rear of truck rises on hard braking
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25. Control Valves, Lines and Hoses, and
Warning Lights
 Lines and Hoses
 Brake lines are made of steel with flared
fittings at each end
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26. Control Valves, Lines and Hoses, and
Warning Lights
 Lines and Hoses
 Flexible hoses connect from fixed location to
caliper or wheel cylinder to allow for
suspension movement
 Two types of brake-line fittings
 Double flare and ISO
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27. Control Valves, Lines and Hoses, and
Warning Lights
 Warning Lights
 Red Brake Warning Light
 Attached to any of the three following
switches or sensors:
 Parking brake
 Brake fluid level sensor
 Diagonal split system warning light
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