2001 TOYOTA PRIUS Service Repair Manual

CHASSIS ± BRAKES107
6. Construction and Operation
The brake system of Prius consists of the following components:
Components
Function
Pump and Pump Motor
Draws up the brake fluid from the reservoir tank
and provides high hydraulic pressure to the
accumulator.
Accumulator
Stores the hydraulic pressure that was generated
by the pump. The accumulator is filled with
highpressure nitrogen gas.
Power
Supply
PortionPressure Switches
Monitors the hydraulic pressure of the
accumulator and outputs control signals for the
pump motor.
There are two types: the pressure switch PH for
controlling the pump, and the pressure switch PL
for giving a warning when the pressure is low.
Hydraulic
Brake
Booster
Relief Valve
Returns the brake fluid to the reservoir tank to
prevent excessive pressure if the pump operates
continuously due to a malfunction of the pressure
switch.
Reservoir TankStores the brake fluid.
Brake Fluid Level
Warning SwitchDetects the low brake fluid level.
Master
Master Cylinder
Generates the hydraulic pressure in accordance
with the pedal effort that is applied to the brake
pedal.
Master
Cylinder
Portion
Brake Booster
Regulates the accumulator pressure in accordance
with the pedal effort that is applied to the brake
pedal and introduces this pressure to the boosterBrake Boosterpedal and introduces this pressure to the booster
chamber in order to provide a power assist to the
brakes.
Pressure SensorsDetects the pressure of the master cylinder,
regulator, and front and rear wheel cylinders.
Switching
Solenoid ValvesSwitches the hydraulic path between normal
braking and braking under control.
Brake
Actuator
ABS Control Solenoid
Valves
Pressure Holding
Valves
Pressure Reduction
Valves
Controls the hydraulic pressure that is applied to
the wheel cylinders during ABS control or EBD
control.
Actuator
Linear Solenoid Valve
Regulates the hydraulic pressure to the wheel
cylinders during braking in accordance with the
fluctuations in the regenerative brake force.
Reservoir
Temporarily stores the brake fluid when
regulating the hydraulic pressure to the wheel
cylinders in accordance with the fluctuations in
the regenerative brake force.
Stroke SimulatorGenerates a pedal stroke during braking in
accordance with the driver's pedal effort.

CHASSIS ± BRAKES
152CH17 182CH47
Reservoir Tank
Accumulator
Check
Valve
Pump and
Pump Motor To Master
Cylinder and
Solenoid
Valves
Pressure
Switch
PL
Relief
Valve
Pressure
Switch
PH
Brake ECUPump Motor
Relays
Brake Warning
BuzzerBrake System
Warning LightAccumulator
Pressure
Pressure
Switch PH
Pressure
Switch PLON
OFF
OFF
Time
Pump
Motor
Relief
ValveOpen
CloseBrake System
Warning Light
and Brake
Warning Buzzer
ON
OFF
ON
OFF
108
Hydraulic Brake Booster
1) General
Due to installation space constraints, a type of hydraulic brake booster that comprises two separate por-
tions has been adopted: the power supply portion (pump, pump motor, accumulator, pressure switches,
and relief valve) and the master cylinder portion.
Consisting of an pump and pump motor, accumulator, and hydraulic pressure switches, the hydraulic
brake booster performs the functions of both the booster and the master cylinder.
By optimizing the hydraulic circuits, the hydraulic pressure that is generated by the booster is applied
directly to the rear brakes. Thus, the hydraulic pressure from the master cylinder is used exclusively by
the front brakes.
2) Pump and Pump Motor, Accumulator, Pressure Switches and Relief Valve
If the accumulator pressure becomes lower than the pressure that is specified in the pressure switch PH,
which is used for detecting high pressure, the pressure switch PH turns OFF. Then, the brake ECU turns
ON the pump motor relays to operate the pump motor and the pump.
The brake fluid that is discharged by the pump passes through the check valve and is stored in the accumu-
lator. The hydraulic pressure that is stored in the accumulator is used for providing the hydraulic pressure
that is needed for normal braking, for operating the ABS, and for operating the EBD control, and for re-
generative brake cooperative controlling.
If the accumulator pressure becomes higher than the pressure that is specified in the pressure switch PH,
the pressure switch PH turns ON. Then, after several seconds, the brake ECU turns OFF the pump.
At this time, if the pressure switch PH malfunctions and causes the pump to operate continuously, the
relief valve opens to prevent excessive pressure from being generated.
Moreover, if the accumulator pressure becomes lower than the pressure that is specified in the pressure
switch PL, which is used for detecting low pressure, the pressure switch PL turns OFF. As a result, the
brake system warning light turns ON and the brake warning buzzer activates.
At this time, the ABS or EBD is prohibited from operating.

CHASSIS ± BRAKES
182CH48
Rubber
Reaction
DiscReaction
RodSpool ValveReturn Spring
RegulatorMaster Cylinder Piston
Power Piston
Operating Rod
Regulator
Master Cylinder Operating Portion
152CH24
Reaction RodTo
ReservoirFrom
Accumulator
Regulator
From
Reservoir Tank
Power Piston
Operating Rod
Rubber Reaction Disc
Spool Valve
Rear Brake Front Brake
Master Cylinder Piston109
2) Master Cylinder and Brake Booster
a. Construction
This construction enables the hydraulic pressure that is generated by the brake booster to be applied
directly to the rear brakes.
The master cylinder is the center port type single master cylinder, which is used for the front brakes
only.
The brake booster is integrated with the master cylinder. The operating portion, master cylinder, and
regulator are positioned coaxially to achieve a simple and compact construction.
The operating rod and the power piston are linked directly to transmit the pedal effort that is applied
to the brake pedal.
The regulator and the spool valve are linked directly. A forward (leftward) force generated by the
master cylinder pressure and a rearward (rightward) force generated by the power assist of the boost-
er are applied to the regulator. Both forces maintain a balance.
A return spring is provided for the regulator to ensure the return of the spool valve.

Cross-Sectional Drawing 


Simplified Drawing