Hydraulic brake control apparatus

A hydraulic brake control apparatus in which a master cylinder supplies a brake fluid pressure responsive to a brake operating force to wheel cylinders through a first valve, includes a first control unit which controls second valves and third valves when a brake fluid pressure supplied to the wheel cylinders by a pressure supplying device is detected to be excessively high under a condition in which the supply of the brake fluid pressure to the wheel cylinders by the master cylinder is inhibited by the first valve, so that the supply of the brake fluid pressure to the wheel cylinders by the pressure supplying device is inhibited by the second valves and a flow of the brake fluid from the wheel cylinders into a reservoir tank is allowed by the third valves. A second control unit controls the third valves and the first valve at a predetermined time after the flow of the brake fluid from the wheel cylinders into the reservoir tank is allowed, so that the flow of the brake fluid from the wheel cylinders into the reservoir tank is inhibited by the third valves and the supply of the brake fluid pressure to the wheel cylinders by the master cylinder is allowed by the first valve.

BACKGROUND OF THE INVENTION 
(1) Field of the Invention 
The present invention generally relates to a hydraulic brake control 
apparatus, and more particularly to a hydraulic brake control apparatus 
which appropriately controls a brake fluid pressure in a brake system of 
an automotive vehicle. 
(2) Description of the Related Art 
A hydraulic brake control apparatus having two pressure supplying devices 
connected to wheel cylinders of a brake system in an automotive vehicle is 
known. For example, Japanese Laid-Open Patent Application No.1-278874 
discloses this type of apparatus. 
In the conventional apparatus of the above-mentioned publication, a master 
cylinder supplies a brake fluid pressure responsive to a brake operating 
force, and a pressure supplying device supplies a brake fluid pressure 
responsive to a drive signal. The pressure supplying device is controlled 
by the drive signal such that the brake fluid pressure supplied by the 
pressure supplying device is set at a brake fluid pressure equivalent to 
the brake operating force multiplied by a predetermined constant. 
The conventional apparatus of the above-mentioned publication includes a 
pressure selection device. The pressure selection device selects a higher 
one of the pressure supplied by the master cylinder and the pressure 
supplied by the pressure supplying device when a brake pedal is depressed 
by the brake operating force by a vehicle operator. The higher pressure 
selected by the pressure selection device is supplied to the wheel 
cylinders. 
The conventional apparatus of the above-mentioned publication includes a 
pressure reduction device. The pressure reduction device reduces the brake 
fluid pressure from the pressure supplying device when the pressure from 
the pressure supplying device is excessively high. The pressure reduction 
device supplies a reduced pressure to the pressure selection device for 
the selection of the brake fluid pressure to be supplied to the wheel 
cylinders. 
According to the conventional apparatus of the above-mentioned publication, 
it is possible to smoothly increase the brake fluid pressure in the wheel 
cylinders by selectively using one of the master cylinder and the pressure 
supplying device after the brake pedal is depressed by the brake operating 
force. In addition, when the pressure supplied by the pressure supplying 
device is excessively high due to a defect in the pressure supplying 
device (this defect will be called the high-pressure defect), the 
conventional apparatus can prevent the supply of the excessively high 
pressure by the defective pressure supplying device directly to the wheel 
cylinders by using the pressure reduction device. Therefore, the 
conventional apparatus enables the brake system to quickly produce a 
braking force responsive to the brake operating force by the vehicle 
operator. Also, the conventional apparatus provides a fail-safe function 
when the high-pressure defect in the pressure supplying device has 
occurred. 
However, the conventional apparatus of the above-mentioned publication 
requires the pressure reduction device in order to provide the fail-safe 
function against the high-pressure defect in the pressure supplying 
device. The production of the pressure reduction device needs a high cost, 
and it is difficult for the conventional apparatus of the above-mentioned 
publication to provide the fail-safe function with a low cost. 
If the supply of the brake fluid pressure to the wheel cylinders by the 
pressure supplying device is inhibited and the supply of the brake fluid 
pressure to the wheel cylinders by the master cylinder is allowed when the 
high-pressure defect in the pressure supplying device has occurred, it is 
possible to control the brake fluid pressure in the wheel cylinders to be 
equal to the brake fluid pressure from the master cylinder. By taking this 
method, it is possible to prevent the brake fluid pressure in the wheel 
cylinders from being increased to the excessively high pressure due to the 
high-pressure defect. 
A flow control device which carries out the above-mentioned method can be 
manufactured with a cost lower than the cost of the pressure reduction 
device. By using the flow control device, the fail-safe function against 
the high-pressure defect may be provided with a low cost. 
However, when the high-pressure defect in the pressure supplying device has 
occurred, the brake fluid pressure in the wheel cylinders at that time is 
higher than the brake fluid pressure in the master cylinder. If the supply 
of the brake fluid pressure by the pressure supplying device is simply 
changed to the supply of the brake fluid pressure by the master cylinder 
when the high-pressure defect has occurred, the high-pressure brake fluid 
from the wheel cylinders may be returned back to the master cylinder. It 
is difficult to avoid the counter flow of the brake fluid into the master 
cylinder even when the above method is utilized, and the counter flow of 
the brake fluid into the master cylinder is detrimental to the durability 
of the master cylinder. 
Accordingly, the above method which changes the supply of the brake fluid 
pressure by the pressure supplying device to the supply of the brake fluid 
pressure by the master cylinder when the high-pressure defect in the 
pressure supplying device has occurred, is not necessarily appropriate to 
provide the fail-safe function against the high-pressure defect. It is 
difficult for the above method to assure the durability of the master 
cylinder and provide the fail-safe function against the high-pressure 
defect. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an improved hydraulic 
brake control apparatus in which the above-described problems are 
eliminated. 
Another object of the present invention is to provide a hydraulic brake 
control apparatus which provides the fail-safe function against the 
high-pressure defect with a low cost and assures the durability of the 
master cylinder. 
The above-mentioned objects of the present invention are achieved by a 
hydraulic brake control apparatus in which a master cylinder supplies a 
brake fluid pressure responsive to a brake operating force to wheel 
cylinders through a first valve, a pressure supplying device supplies a 
brake fluid pressure responsive to a drive signal to the wheel cylinders 
through second valves, and the wheel cylinders are connected to a 
reservoir tank via third valves, the apparatus comprising: a first control 
unit which controls the second valves and the third valves when the brake 
fluid pressure supplied to the wheel cylinders by the pressure supplying 
device is detected to be excessively high under a condition in which the 
supply of the brake fluid pressure to the wheel cylinders by the master 
cylinder is inhibited by the first valve, so that the supply of the brake 
fluid pressure to the wheel cylinders by the pressure supplying device is 
inhibited by the second valves and a flow of the brake fluid from the 
wheel cylinders into the reservoir tank is allowed by the third valves; 
and a second control unit which controls the third valves and the first 
valve at a predetermined time after the flow of the brake fluid from the 
wheel cylinders into the reservoir tank is allowed, so that the flow of 
the brake fluid from the wheel cylinders into the reservoir tank is 
inhibited by the third valves and the supply of the brake fluid pressure 
to the wheel cylinders by the master cylinder is allowed by the first 
valve. 
In the hydraulic brake control apparatus of the present invention, when the 
high-pressure defect in the pressure supplying device has occurred, the 
supply of the brake fluid pressure to the wheel cylinders by the pressure 
supplying device is changed to the supply of the brake fluid pressure to 
the wheel cylinders by the master cylinder without producing the counter 
flow of the brake fluid into the master cylinder. Therefore, according to 
the hydraulic brake control apparatus of the present invention, it is 
possible to provide the fail-safe function against the high-pressure 
defect in the pressure supplying device and assure the durability of the 
master cylinder. 
In addition, according to the hydraulic brake control apparatus of the 
present invention, it is possible to effectively prevent the reduction of 
the braking force on the vehicle wheels by the wheel cylinders in the 
course of the change from the supply of the brake fluid pressure by the 
pressure supplying device to the supply of the brake fluid pressure by the 
master cylinder upon occurrence of the high-pressure defect in the 
pressure supplying device. Therefore, the hydraulic brake control 
apparatus of the present invention enables the brake system to maintain an 
adequately great brake force on the vehicle wheels even when the 
high-pressure defect in the pressure supplying device has occurred.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A description will now be given of the preferred embodiments of the present 
invention with reference to the accompanying drawings. 
FIG. 1 shows a brake system to which a hydraulic brake control apparatus of 
the present invention is applied. 
As shown in FIG. 1, the brake system is used in an automotive vehicle and 
includes an electronic control unit (ECU) 10. The electronic control unit 
10 controls the elements of the brake system in accordance with the 
operating conditions of the vehicle. Hereinafter, the electronic control 
unit 10 will be called the ECU 10. 
In FIG. 1, input signal paths over which signals from the elements of the 
brake system are sent to the ECU 10, and output signal paths over which 
signals from the ECU 10 are sent to the elements of the brake system are 
omitted for the sake of convenience. Unless otherwise specified, these 
signal paths are indicated by dotted-line arrows in FIG. 1, and brake 
fluid paths on which brake fluid is supplied between the elements of the 
brake system are indicated by solid lines in FIG. 1. 
The brake system comprises a brake pedal 12. The brake pedal 12 is 
connected to an input shaft 16 of a master cylinder 14. 
The master cylinder 14 includes a first piston 18 and a second piston 20 
which are provided in the master cylinder 14. The first piston 18 is 
connected to the input shaft 16 via a front surface of the master cylinder 
14. In the master cylinder 14, a first pressure chamber 22 between the 
first piston 18 and the second piston 20 is provided, and a second 
pressure chamber 24 between the second piston 20 and a rear surface of the 
master cylinder 14 is provided. In the second pressure chamber 24, a 
spring 26 is provided, and the spring 26 exerts an actuating force on the 
second piston 20 to push the second piston 20 toward the brake pedal 12. 
When the brake pedal 12 is set at an original position (or the brake pedal 
12 is not depressed by the vehicle operator), the first piston 18 and the 
second piston 20 in the master cylinder 14 are set at starting positions. 
When the brake pedal 12 is set at an operated position (or the brake pedal 
12 is depressed by the vehicle operator) and then released, the first 
piston 18 and the second piston 20 are returned to the starting positions. 
A reservoir tank 28 is provided above the master cylinder 14. The reservoir 
tank 28 contains a brake fluid stored therein. The reservoir tank 28 is 
connected to both the first pressure chamber 22 and the second pressure 
chamber 24 only when the first piston 18 and the second piston 20 are set 
at the starting positions in the master cylinder 14. Otherwise the 
reservoir tank 28 is disconnected from the master cylinder 14. Therefore, 
the brake fluid in the reservoir tank 28 is replenished to both the first 
pressure chamber 22 and the second pressure chamber 24 of the master 
cylinder 14 each time the brake pedal 12 is released by the vehicle 
operator or set at the original position. 
A master cylinder pressure sensor 30 (which will be called the Pmc sensor 
30) and a pressure switch 32 are provided in the second pressure chamber 
24 of the master cylinder 14. 
The Pmc sensor 30 outputs a signal indicative of a brake fluid pressure in 
the second pressure chamber 24 of the master cylinder 14. The signal 
output from the Pmc sensor 30 is supplied to the ECU 10. The ECU 10 
detects a master cylinder pressure Pmc (which will be called the pressure 
Pmc) based on the signal from the Pmc sensor 30. 
The pressure switch 32 outputs an ON signal when the brake fluid pressure 
in the master cylinder 14 is higher than a reference level. The ON signal 
from the pressure switch 32 is supplied to the ECU 10. The ECU 10 detects 
whether the brake pedal 12 is set at the operated position (or whether it 
is depressed by the vehicle operator), based on whether the ON signal from 
the pressure switch 32 is supplied to the ECU 10. 
A pressure passage 34 is connected at one end to the first pressure chamber 
22 of the master cylinder 14. The pressure passage 34 is connected at the 
other end to a pressure passage 40 via a master cylinder cut valve 36 
(which will be called the MCV 36). In addition, the pressure passage 34 is 
connected to the pressure passage 40 via a bypass passage in which a check 
valve 38 is provided. 
The MCV 36 is an electromagnetic flow control valve which is electrically 
set at one of an opened position and a closed position. When a drive 
signal from the ECU 10 is not supplied to the MCV 36, the MCV 36 is always 
at the opened position so that the brake fluid from the master cylinder 14 
is allowed to flow into the pressure passage 40 via the MCV 36. When the 
drive signal from the ECU 10 is supplied to the MCV 36, the MCV 36 is 
electrically set at the closed position, so that a flow of the brake fluid 
from the master cylinder 14 into the pressure passage 40 via the MCV 36 is 
inhibited. 
The check valve 38 is a type of valve which allows a flow of fluid in one 
direction only. The check valve 38 allows the flow of the brake fluid from 
the pressure passage 34 to the pressure passage 40 via the bypass passage. 
The check valve 38 inhibits the flow of the brake fluid in the opposite 
direction. 
The brake system comprises a pump 42. The pump 42 is provided with an 
actuating motor 44. The actuating motor 44 actuates the pump 42 so that 
the pump 42 supplies a high-pressure brake fluid. The operation of the 
motor 44 is controlled by the ECU 10. An inlet port of the pump 42 is 
connected to the reservoir tank 28. An outlet port of the pump 42 is 
connected to a high-pressure passage 50 via a check valve 48. An 
accumulator 46 is provided between the outlet port of the pump 42 and the 
check valve 48, and the accumulator 46 and the pump 42 connected to each 
other. 
An accumulator pressure sensor 52 (which will be called the Pacc sensor 52) 
is connected to the high-pressure passage 50. The Pacc sensor 52 outputs a 
signal indicative of a brake fluid pressure in the high-pressure passage 
50. The signal output from the Pacc sensor 52 is supplied to the ECU 10. 
The ECU 10 detects the brake fluid pressure in the high-pressure passage 
50 based on the signal from the Pacc sensor 52. An accumulator pressure 
Pacc (which will be called the pressure Pacc) in the accumulator 46 is 
equivalent to the brake fluid pressure in the high-pressure passage 50. 
That is, the ECU 10 detects the present level of the pressure Pacc in the 
accumulator 46 based on the signal from the Pacc sensor 52. 
An upper-limit sensor 54 and a lower-limit sensor 56 are connected to the 
high-pressure passage 50 at positions adjacent to the Pacc sensor 52. The 
upper-limit sensor 54 outputs an ON signal when the brake fluid pressure 
(or the pressure Pacc) in the high-pressure passage 50 is higher than an 
upper limit of a predetermined operating pressure range of the pressures 
Pacc for the accumulator 46. The lower-limit sensor 56 outputs an ON 
signal when the brake fluid pressure in the high-pressure passage 50 is 
lower than a lower limit of the predetermined operating pressure range of 
the pressures Pacc for the accumulator 46. Both the signal from the 
upper-limit sensor 54 and the signal from the lower-limit sensor 56 are 
supplied to the ECU 10. 
When the lower-limit sensor 56 outputs an ON signal to the ECU 10, the ECU 
10 supplies a drive signal to the motor 44 so that the pump 42 is actuated 
by the motor 44. The ECU 10 continues to supply the drive signal to the 
motor 44 until the upper-limit sensor 54 outputs an ON signal to the ECU 
10. According to this operation, the pressure Pacc in the accumulator 46 
is always maintained to be in the predetermined operating pressure range. 
A pressure-up linear valve 58 and a pressure-up linear valve 60 are 
connected to the high-pressure passage 50 at the end of the high-pressure 
passage 50. The pressure-up linear valve 58 is connected at the other end 
to a front pressure passage 62, and the pressure-up linear valve 60 is 
connected at the other end to a rear pressure passage 64. 
The front pressure passage 62 is connected to the reservoir tank 28 via a 
first return passage 67 in which a pressure-down linear valve 66 is 
provided. The rear pressure passage 64 is connected to the reservoir tank 
28 via a second return passage 69 in which a pressure-down linear valve 68 
is provided. 
When drive signals are not supplied by the ECU 10, the pressure-up linear 
valves 58 and 60 and the pressure-down linear valves 66 and 68 are set at 
the closed positions. When the drive signals are supplied by the ECU 10, 
the pressure-up linear valves 58 and 60 and the pressure-down linear 
valves 66 and 68 are set at the opened positions. When one of the four 
linear valves 58, 60, 66 and 68 is set at the opened position, an 
effective valve-opening area in a corresponding one of the passages 62, 
64, 67 and 69 is formed by the related linear valve. The effective 
valve-opening area formed by the related linear valve varies in proportion 
to the level of the drive signal supplied by the ECU 10. 
Therefore, in the above-described hydraulic brake control apparatus, by 
changing the level of the drive signal supplied to the pressure-up linear 
valve 58, a pressure of the brake fluid fed from the high-pressure passage 
50 into the front pressure passage 62 can be controlled such that the 
brake fluid pressure is linearly changed. By changing the level of the 
drive signal supplied to the pressure-down linear valve 66, a pressure of 
the brake fluid fed from the front pressure passage 62 into the reservoir 
tank 28 can be controlled such that the brake fluid pressure is linearly 
changed. By changing the level of the drive signal supplied to the 
pressure-up linear valve 60, a pressure of the brake fluid fed from the 
high-pressure passage 50 into the rear pressure passage 64 can be 
controlled such that the brake fluid pressure is linearly changed. By 
changing the level of the drive signal supplied to the pressure-down 
linear valve 68, a pressure of the brake fluid fed from the rear pressure 
passage 64 into the reservoir tank 28 can be controlled such that the 
brake fluid pressure is linearly changed. 
A front cut valve 70 (which will be called the FCV 70) is connected to the 
front pressure passage 62 at the end of the front pressure passage 62. The 
FCV 70 is connected at the other end to a front hydraulic circuit 72. 
The FCV 70 is an electromagnetic flow control valve which is set at one of 
an opened position and a closed position. When a drive signal from the ECU 
10 is not supplied to the FCV 70, the FCV 70 is always at the closed 
position so that a flow of the brake fluid from the front pressure passage 
62 into the front hydraulic circuit 72 via the FCV 70 is inhibited. When 
the drive signal from the ECU 10 is supplied to the FCV 70, the FCV 70 is 
electrically set at the opened position, so that the brake fluid from the 
front pressure passage 62 is allowed to flow into the front hydraulic 
circuit 72 via the FCV 70. 
In addition, the pressure passage 40 is connected to the front hydraulic 
circuit 72 via a connection passage 76. When the MCV 36 is set at the 
opened position, the master cylinder 14 and the front hydraulic circuit 72 
are connected to each other via the pressure passage 40. In this 
condition, the brake fluid whose pressure is equal to the pressure Pmc is 
supplied from the master cylinder 14 to the front hydraulic circuit 72. 
When the MCV 36 is set at the closed position, the master cylinder 14 and 
the front hydraulic circuit 72 are connected to each other via the bypass 
passage in which the check valve 38 is provided. In this condition, if the 
pressure Pmc is higher than a brake fluid pressure in the front hydraulic 
circuit 72, the brake fluid pressure in the front hydraulic circuit 72 is 
increased to the pressure Pmc. This brake fluid pressure will be equal to 
the pressure Pmc. If the pressure Pmc is lower than the brake fluid 
pressure in the front hydraulic circuit 72, the brake fluid pressure in 
the front hydraulic circuit 72 is retained at the same level. This brake 
fluid pressure is different from the pressure Pmc. 
A front wheel cylinder pressure sensor 74 (which will be called the Pf 
sensor 74) is connected to the connection passage 76 which interconnects 
the FCV 70 and the front hydraulic circuit 72. The Pf sensor 74 
communicates with the pressure passage 40 via the connection passage 76. 
The Pf sensor 74 outputs a signal indicative of a brake fluid pressure in 
the connection passage 76. The signal output from the Pf sensor 74 is 
supplied to the ECU 10. The ECU 10 detects the present level of the brake 
fluid pressure in the connection passage 76 based on the signal from the 
Pf sensor 74. 
The front hydraulic circuit 72 includes a pressure hold valve 78 and a 
check valve 80 which are provided in parallel. The pressure hold valve 78 
and the check valve 80 are connected to a wheel cylinder 82 provided for a 
front left ("FL") wheel of the vehicle. The connection passage 76 is 
connected to the wheel cylinder 82 via the pressure hold valve 78 and the 
check valve 80. 
The check valve 80 allows the flow of the brake fluid from the wheel 
cylinder 82 to the connection passage 76. The check valve 80 inhibits the 
flow of the brake fluid from the connection passage 76 to the wheel 
cylinder 82. The pressure hold valve 78 is an electromagnetic flow control 
valve which is set at one of an opened position and a closed position. 
When a drive signal from the ECU 10 is not supplied to the pressure hold 
valve 78, the pressure hold valve 78 is always at the opened position so 
that the brake fluid from the connection passage 76 is allowed to flow 
into the wheel cylinder 82 via the pressure hold valve 78. When the drive 
signal from the ECU 10 is supplied to the pressure hold valve 78, the 
pressure hold valve 78 is electrically set at the closed position, so that 
the flow of the brake fluid from the connection passage 76 to the wheel 
cylinder 82 via the pressure hold valve 78 is inhibited. 
The front hydraulic circuit 72 includes a pressure hold valve 84 and a 
check valve 86 which are provided in parallel. The pressure hold valve 84 
and the check valve 86 are connected to a wheel cylinder 88 provided for a 
front right ("FR") wheel of the vehicle. The connection passage 76 is 
connected to the wheel cylinder 88 via the pressure hold valve 84 and the 
check valve 86. 
The check valve 86 allows the flow of the brake fluid from the wheel 
cylinder 88 to the connection passage 76. The check valve 86 inhibits the 
flow of the brake fluid from the connection passage 76 to the wheel 
cylinder 88. The pressure hold valve 84 is an electromagnetic flow control 
valve which is set at one of an opened position and a closed position. 
When a drive signal from the ECU 10 is not supplied to the pressure hold 
valve 84, the pressure hold valve 84 is always at the opened position so 
that the brake fluid from the connection passage 76 is allowed to flow 
into the wheel cylinder 88 via the pressure hold valve 84. When the drive 
signal from the ECU 10 is supplied to the pressure hold valve 84, the 
pressure hold valve 84 is electrically set at the closed position, so that 
the flow of the brake fluid from the connection passage 76 to the wheel 
cylinder 88 via the pressure hold valve 84 is inhibited. 
The wheel cylinder 82 is connected to a low-pressure passage 94 via a 
pressure-down valve 90, and the wheel cylinder 88 is connected to the 
low-pressure passage 94 via a pressure-down valve 92. Each of the 
pressure-down valves 90 and 92 is an electromagnetic flow control valve 
which is electrically set at one of an opened position and a closed 
position. When signals from the ECU 10 are not supplied, the pressure-down 
valves 90 and 92 are always at the closed positions, so that the flow of 
the brake fluid from the wheel cylinders 82 and 88 into the low-pressure 
passage 94 via the pressure-down valves 90 and 92 is inhibited. When 
signals from the ECU 10 are supplied, the pressure-down valves 90 and 92 
are set at the opened positions, so that the flow of the brake fluid from 
the wheel cylinders 82 and 88 into the low-pressure passage 94 via the 
pressure-down valves 90 and 92 is allowed. 
A reservoir cut valve 96 (which will be called the RVCV 96) is connected to 
the low-pressure passage 94 at the end of the low-pressure passage 94. The 
RVCV 96 is connected at the other end to the reservoir tank 28 via a 
return passage 97. The RVCV 96 is an electromagnetic flow control valve 
which is electrically set at one of an opened position and a closed 
position. When a drive signal from the ECU 10 is not supplied, the RVCV 96 
is always at the closed position so that the flow of the brake fluid from 
the low-pressure passage 94 into the reservoir tank 28 via the RVCV 96 is 
inhibited. When the drive signal from the ECU 10 is supplied, the RVCV 96 
is set at the opened position, so that the brake fluid from the 
low-pressure passage 94 is allowed to flow into the reservoir tank 28 via 
the RVCV 96. 
A rear wheel cylinder pressure sensor 98 (which will be called the Pr 
sensor 98) is connected to the rear pressure passage 64. The Pr sensor 98 
outputs a signal indicative of a brake fluid pressure in the rear pressure 
passage 64. The signal output from the Pr sensor 98 is supplied to the ECU 
10. The ECU 10 detects the present level of the brake fluid pressure in 
the rear pressure passage 64 based on the signal from the Pr sensor 98. 
A rear cut valve 100 (which will be called the RCV 100) is connected to the 
rear pressure passage 64 at the end of the rear pressure passage 64. The 
RCV 100 is connected at the other end to a rear hydraulic circuit 102 via 
a connection passage 104. 
The RCV 100 is an electromagnetic flow control valve which is electrically 
set at one of an opened position and a closed position. When a drive 
signal from the ECU 10 is not supplied to the RCV 100, the RCV 100 is 
always at the closed position so that the flow of the brake fluid from the 
rear pressure passage 64 into the rear hydraulic circuit 102 via the RCV 
100 is inhibited. When the drive signal from the ECU 10 is supplied to the 
RCV 100, the RCV 100 is electrically set at the opened position, so that 
the brake fluid from the rear pressure passage 64 is allowed to flow into 
the rear hydraulic circuit 102 via the RCV 100. 
The rear hydraulic circuit 102 includes a pressure hold valve 106 and a 
check valve 108 which are provided in parallel. The pressure hold valve 
106 and the check valve 108 are connected to a wheel cylinder 110 provided 
for a rear left ("RL") wheel of the vehicle. The connection passage 104 is 
connected to the wheel cylinder 110 via the pressure hold valve 106 and 
the check valve 108. 
The check valve 108 allows the flow of the brake fluid from the wheel 
cylinder 110 to the connection passage 104. The check valve 108 inhibits 
the flow of the brake fluid from the connection passage 104 to the wheel 
cylinder 110. The pressure hold valve 106 is an electromagnetic flow 
control valve which is electrically set at one of an opened position and a 
closed position. When a drive signal from the ECU 10 is not supplied to 
the pressure hold valve 106, the pressure hold valve 106 is always at the 
opened position so that the brake fluid from the connection passage 104 is 
allowed to flow into the wheel cylinder 110 via the pressure hold valve 
106. When the drive signal from the ECU 10 is supplied to the pressure 
hold valve 106, the pressure hold valve 106 is electrically set at the 
closed position, so that the flow of the brake fluid from the connection 
passage 104 to the wheel cylinder 110 via the pressure hold valve 106 is 
inhibited. 
The rear hydraulic circuit 102 includes a pressure hold valve 112 and a 
check valve 114 which are provided in parallel. The pressure hold valve 
112 and the check valve 114 are connected to a wheel cylinder 116 provided 
for a rear right ("RR") wheel of the vehicle. The connection passage 104 
is connected to the wheel cylinder 116 via the pressure hold valve 112 and 
the check valve 114. 
The check valve 114 allows the flow of the brake fluid from the wheel 
cylinder 116 to the connection passage 104. The check valve 114 inhibits 
the flow of the brake fluid from the connection passage 104 to the wheel 
cylinder 116. The pressure hold valve 112 is an electromagnetic flow 
control valve which is electrically set at one of an opened position and a 
closed position. When a drive signal from the ECU 10 is not supplied to 
the pressure hold valve 112, the pressure hold valve 112 is always at the 
opened position so that the brake fluid from the connection passage 104 is 
allowed to flow into the wheel cylinder 116 via the pressure hold valve 
112. When the drive signal from the ECU 10 is supplied to the pressure 
hold valve 112, the pressure hold valve 112 is electrically set at the 
closed position, so that the flow of the brake fluid from the connection 
passage 104 to the wheel cylinder 116 via the pressure hold valve 112 is 
inhibited. 
The wheel cylinder 110 is connected to the low-pressure passage 94 via a 
pressure-down valve 118, and the wheel cylinder 116 is connected to the 
low-pressure passage 94 via a pressure-down valve 120. Each of the 
pressure-down valves 118 and 120 is an electromagnetic flow control valve 
which is electrically set at one of an opened position and a closed 
position. When signals from the ECU 10 are not supplied, the pressure-down 
valves 118 and 120 are always at the closed positions, so that the flow of 
the brake fluid from the wheel cylinders 110 and 116 into the low-pressure 
passage 94 via the pressure-down valves 118 and 120 is inhibited. When 
signals from the ECU 10 are supplied, the pressure-down valves 118 and 120 
are set at the opened positions, so that the flow of the brake fluid from 
the wheel cylinders 110 and 116 into the low-pressure passage 94 via the 
pressure-down valves 118 and 120 is allowed. 
In the brake system of FIG. 1, when a brake operating force Fp by the 
vehicle operator is placed onto the brake pedal 12, the brake fluid 
pressure in the first pressure chamber 22 and the brake fluid pressure in 
the second pressure chamber 24 are increased in response to the brake 
operating force Fp on the brake pedal 12. 
When the brake fluid pressure in the second pressure chamber 24 of the 
master cylinder 14 is higher than the reference level, the pressure switch 
32 outputs an ON signal to the ECU 10. The ECU 10 detects that the brake 
pedal 12 is set at the operated position or depressed by the vehicle 
operator, based on the ON signal from the pressure switch 32. 
When the depression of the brake pedal 12 by the vehicle operator is 
detected, the ECU 10 supplies signals to the MCV 36, the FCV 70 and the 
RCV 10, so that the MCV 36 is set at the closed position, and the FCV 70 
and the RCV 100 are set at the opened positions. 
In this case, the flow of the brake fluid from the master cylinder 14 into 
the pressure passage 40 via the MCV 36 is inhibited. The brake fluid from 
the front pressure passage 62 is allowed to flow into the front hydraulic 
circuit 72 via the FCV 70. The pressure of the brake fluid supplied from 
the FCV 70 is adjusted to a pressure Pf by the pressure-up linear valve 58 
and the pressure-down linear valve 66, and these linear valves 58 and 66 
will be called the front linear valves 58 and 66. Further, the brake fluid 
from the rear pressure passage 64 is allowed to flow into the rear 
hydraulic circuit 102 via the RCV 100. The pressure of the brake fluid 
supplied from the RCV 100 is adjusted to a brake fluid pressure Pr by the 
pressure-up linear valve 60 and the pressure-down linear valve 68, and 
these linear valves 60 and 68 will be called the rear linear valves 60 and 
68. 
Under such conditions, the Pmc sensor 30 outputs a signal indicative of the 
pressure Pmc in the master cylinder 14 in response to the brake operating 
force Fp. The Pf sensor 74 outputs a signal indicative of the pressure Pf 
in the connection passage 76, the pressure Pf being adjusted by the front 
linear valves 58 and 66. The Pr sensor 98 outputs a signal indicative of 
the pressure Pr in the rear pressure passage 64 or the connection passage 
104, the pressure Pr being adjusted by the rear linear valves 60 and 68. 
Based on the signals from the Pmc sensor 30 and the Pf sensor 74, the ECU 
10 controls the front linear valves 58 and 66 such that a brake fluid 
pressure Pf is supplied by the front linear valves 58 and 66 and a ratio 
of the pressure Pf to the pressure Pmc is set at a predetermined constant. 
Based on the signals from the Pmc sensor 30 and the Pr sensor 98, the ECU 
10 controls the rear linear valves 60 and 68 such that a brake fluid 
pressure Pr is supplied by the rear linear valves 60 and 68 and a ratio of 
the pressure Pr to the pressure Pmc is set at a predetermined constant. 
When a hydraulic brake control process is not performed by the hydraulic 
brake control apparatus, the pressure hold valves 78 and 84 are set at the 
opened positions and the pressure-down valves 90 and 92 are set at the 
closed positions. Under such conditions, the brake fluid pressure Pf in 
the connection passage 76 of the front hydraulic circuit 72 is supplied to 
the front wheel cylinders 82 and 88. The brake fluid pressure Pf in the 
front wheel cylinders 82 and 88 is adjusted such that a ratio of the 
pressure Pf to the master cylinder pressure Pmc (which varies in response 
to the brake operating force Fp) is set at a predetermined constant. 
Similarly, when the hydraulic brake control process is not performed by the 
hydraulic brake control apparatus, the pressure hold valves 106 and 112 
are set at the opened positions and the pressure-down valves 118 and 120 
are set at the closed positions. Under such conditions, the brake fluid 
pressure Pr in the connection passage 104 of the rear hydraulic circuit 
102 is supplied to the rear wheel cylinders 110 and 116. The brake fluid 
pressure Pr in the rear wheel cylinders 110 and 116 is adjusted such that 
a ratio of the pressure Pr to the master cylinder pressure Pmc (which 
varies in response to the brake operating force Fp) is set at a 
predetermined constant. 
Accordingly, in the brake system of FIG. 1, it is possible to adjust the 
pressure Pf in the front wheel cylinders 82 and 88 and the pressure Pr in 
the rear wheel cylinders 110 and 116 such that the pressure Pf and the 
pressure Pr are set at predetermined values in response to the brake 
operating force Fp. A control procedure in which the above-described 
function is achieved by the brake system when the hydraulic brake control 
process is not performed will be called a normal control mode. 
In the hydraulic brake control apparatus of the above-described embodiment, 
the front linear valves 58 and 66 can be controlled such that the ratio of 
the pressure Pf to the pressure Pmc is set at a predetermined constant 
which is varied independently of the pressure Pmc in the master cylinder 
14, and the rear linear valves 60 and 68 can be controlled such that the 
ratio of the pressure Pr to the pressure Pmc is set at a predetermined 
constant which is varied independently of the pressure Pmc in the master 
cylinder 14. The pressure Pf from the front linear valves 58 and 66 is 
supplied to the connection passage 76 of the front hydraulic circuit 72 
via the FCV 70, and the pressure Pr from the rear linear valves 60 and 68 
is supplied to the connection passage 114 of the rear hydraulic circuit 
102 via the RCV 100. 
Therefore, in the hydraulic brake control apparatus of the above-described 
embodiment, the pressure Pf in the front wheel cylinders 82 and 88 can be 
suitably increased by controlling the front linear valves 58 and 66, and 
the pressure Pr in the rear wheel cylinders 110 and 116 can be suitably 
increased by controlling the rear linear valves 60 and 68. A control 
procedure in which the above-described function is achieved by the 
hydraulic brake control apparatus of the present embodiment will be called 
a pressure-increase control mode. 
Further, in the hydraulic brake control apparatus of the above-described 
embodiment, when the pressure hold valve 78 is set at the closed position 
and the pressure-down valve 90 is set at the closed position, it is 
possible to continuously hold the pressure Pf in the wheel cylinder 82 at 
the same level. When the pressure hold valve 84 is set at the closed 
position and the pressure-down valve 92 is set at the closed position, it 
is possible to hold the pressure Pf in the wheel cylinder 88 at the same 
level. When the pressure hold valve 106 is set at the closed position and 
the pressure-down valve 118 is set at the closed position, it is possible 
to continuously hold the pressure Pr in the wheel cylinder 110 at the same 
level. When the pressure hold valve 112 is set at the closed position and 
the pressure-down valve 120 is set at the closed position, it is possible 
to continuously hold the pressure Pr in the wheel cylinder 116 at the same 
level. 
Therefore, in the hydraulic brake control apparatus of the above-described 
embodiment, the pressure Pf in the front wheel cylinders 82 and 88 can be 
maintained at a desired level by controlling the pressure hold valves 78 
and 84 and the pressure-down valves 90 and 92, and the pressure Pr in the 
rear wheel cylinders 110 and 116 can be maintained at a desired level by 
controlling the pressure hold valves 106 and 112 and the pressure-down 
valves 118 and 120. A control procedure in which the above-described 
function is achieved by the hydraulic brake control apparatus of the 
present embodiment will be called a pressure hold control mode. 
Further, in the hydraulic brake control apparatus of the above-described 
embodiment, when the pressure hold valve 78 is set at the closed position 
and the pressure-down valve 90 and the RVCV 96 are set at the opened 
positions, it is possible to suitably reduce the pressure Pf in the wheel 
cylinder 82. When the pressure hold valve 84 is set at the closed position 
and the pressure-down valve 92 and the RVCV 96 are set at the opened 
positions, it is possible to suitably reduce the pressure Pf in the wheel 
cylinder 88. When the pressure hold valve 106 is set at the closed 
position and the pressure-down valve 118 and the RVCV 96 are set at the 
opened positions, it is possible to suitably reduce the pressure Pr in the 
wheel cylinder 110. When the pressure hold valve 112 is set at the closed 
position and the pressure-down valve 120 and the RVCV 96 are set at the 
closed positions, it is possible to suitably reduce the pressure Pr in the 
wheel cylinder 116. 
Therefore, in the hydraulic brake control apparatus of the present 
embodiment, the pressure Pf in the front wheel cylinders 82 and 88 can be 
suitably reduced by controlling the pressure hold valves 78 and 84, the 
RVCV 96 and the pressure-down valves 90 and 92, and the pressure Pr in the 
rear wheel cylinders 110 and 116 can be suitably reduced by controlling 
the pressure hold valves 106 and 112, the RVCV 96 and the pressure-down 
valves 118 and 120. A control procedure in which the above-described 
function is achieved by the hydraulic brake control apparatus of the 
present embodiment will be called a pressure-decrease control mode. 
The ECU 10 suitably carries out one of the normal control mode, the 
pressure-increase control mode, the pressure hold control mode and the 
pressure-decrease control mode of the brake system based on the operating 
conditions of the vehicle and the respective slip ratios of the FL, FR, RL 
and RR wheels of the vehicle. The normal control mode is carried out in 
order to achieve a hydraulic brake control function responsive to the 
brake operating force of the vehicle operator. The pressure-increase 
control mode, the pressure hold control mode and the pressure-decrease 
control mode in combination are carried out in order to achieve a 
hydraulic brake control function appropriate to ensure a vehicle running 
stability. 
In the hydraulic brake control apparatus of the above-described embodiment, 
if a defect in the front linear valves 58 and 66 has occurred, the 
pressure Pf in the front wheel cylinders 82 and 88 cannot be increased to 
an adequately high pressure due to an excessively low pressure of the 
brake fluid supplied from the defective front linear valves 58 and 66. 
This defect will be called the low-pressure defect. Further, in the 
hydraulic brake control apparatus of the above-described embodiment, if 
another type of defect in the front linear valves 58 and 66 has occurred, 
the pressure Pf in the front wheel cylinders 82 and 88 cannot suitably be 
maintained at the adequate pressure due to an excessively high brake fluid 
pressure supplied from the defective front linear valves 58 and 66. This 
defect will be called the high-pressure defect. 
In order to avoid the above problem, in the hydraulic brake control 
apparatus of the above-described embodiment, when the occurrence of the 
low-pressure defect or the high-pressure defect in the front linear valves 
58 and 66 is detected, the ECU 10 sets the MCV 36 at the opened position 
and sets the FCV 70 at the closed position. The pressure Pf in the front 
wheel cylinders 82 and 88 can be increased to or maintained at the 
adequate pressure by supplying the brake fluid pressure Pmc from the 
master cylinder 14 to the front hydraulic circuit 72 through the MCV 36 
instead of the supply of the brake fluid pressure from the defective 
linear valves 58 and 66. 
Therefore, in the hydraulic brake control apparatus of the above-described 
embodiment, when the occurrence of the low-pressure defect or the 
high-pressure defect in the front linear valves 58 and 66 is detected, it 
is possible to supply the brake fluid pressure Pmc from the master 
cylinder 14, which pressure is responsive to the brake operating force Fp, 
to the wheel cylinders 82 and 88. Therefore, it is possible that the 
hydraulic brake control apparatus of the above-described embodiment safely 
produce an adequately great braking force on the front wheels of the 
vehicle, and the braking force thus produced is responsive to the brake 
operating force Fp even when the low-pressure defect or the high-pressure 
defect in the front linear valves 58 and 66 has occurred. 
When the high-pressure defect in the front linear valves 58 and 66 has 
occurred, the brake fluid pressure Pf in the wheel cylinders 82 and 88 at 
that time is higher than the brake fluid pressure Pmc in the master 
cylinder 14. If the supply of the brake fluid pressure to the wheel 
cylinders 82 and 88 by the front linear valves 58 and 66 is changed to the 
supply of the brake fluid pressure to the wheel cylinders 82 and 88 by the 
master cylinder 14 after the high-pressure defect has occurred, the 
high-pressure brake fluid Pf from the wheel cylinders 82 and 88 may be 
returned back to the master cylinder 14 through the MCV 36. This counter 
flow of the brake fluid is detrimental to the durability of the master 
cylinder 14. 
In order to assure the durability of the master cylinder 14, it is 
desirable to avoid the counter flow of the brake fluid into the master 
cylinder 14 when the high-pressure defect in the front linear valves 58 
and 66 has occurred. In the hydraulic brake control apparatus of the 
present embodiment, when the occurrence of the high-pressure defect in the 
front linear valves 58 and 66 is detected, the supply of the brake fluid 
pressure to the wheel cylinders 82 and 88 by the front linear valves 58 
and 66 is changed to the supply of the brake fluid pressure to the wheel 
cylinders 82 and 88 by the master cylinder 14, and, at the same time, the 
above counter flow of the brake fluid into the master cylinder 14 is 
avoided. 
The above-mentioned function of the hydraulic brake control apparatus of 
the present embodiment is achieved by carrying out a hydraulic brake 
control routine by means of the ECU 10 of the brake system of FIG. 1, 
which will be described in the following. 
FIGS. 2A and 2B show a hydraulic brake control routine performed by a 
hydraulic brake control apparatus embodying the present invention. The 
control routine of FIGS. 2A and 2B is executed by the ECU 10 of the brake 
system of FIG. 1. The control routine of FIGS. 2A and 2B is executed in 
order to carry out the normal control mode when the high-pressure defect 
in the front linear valves 58 and 66 does not occur, and to carry out the 
above-mentioned function when the high-pressure defect in the front linear 
valves 58 and 66 has occurred. 
As shown in FIG. 2A, the ECU 10 at step 200 detects whether the brake pedal 
12 is depressed by the brake operating force by the vehicle operator. As 
described above, when the ON signal from the pressure switch 32 is 
supplied to the ECU 10 or when the pressure Pmc detected by the Pmc sensor 
30 is higher than a reference level, it is detected that the brake pedal 
12 is depressed by the brake operating force by the vehicle operator. 
When the result at the step 200 is negative, the subsequent steps are not 
performed and the control routine of FIGS. 2A and 2B at the present cycle 
ends. When the result at the step 200 is affirmative, step 202 is executed 
by the ECU 10. 
Step 202 sets the MCV 36 at the closed position, sets the FCV 70 at the 
opened position, and sets the RCV 100 at the opened position. This step is 
executed to carry out the normal control mode. After the step 202 is 
performed, step 204 is executed by the ECU 10. 
Step 204 controls the front linear valves 58 and 66 such that the pressure 
Pf (detected by the Pf sensor 74) is supplied by the front linear valves 
58 and 66 and a ratio of the pressure Pf to the pressure Pmc (detected by 
the Pmc sensor 30) is set at a predetermined constant .alpha..sub.f0. 
Further, step 204 controls the rear linear valves 60 and 68 such that the 
pressure Pr (detected by the Pr sensor 98) is supplied by the rear linear 
valves 60 and 68 and a ratio of the pressure Pr to the pressure Pmc is set 
at a predetermined constant .alpha..sub.r0. After the step 204 is 
performed, the equations Pf=.alpha..sub.f0.Pmc and Pr=.alpha..sub.r0.Pmc 
are satisfied. 
In the above step 204, the brake fluid pressure Pf which is equal to the 
master cylinder pressure Pmc multiplied by the constant .alpha..sub.f0, 
can be supplied to the wheel cylinders 82 and 88 provided for the front 
left (FL) wheel and the front right (FR) wheel of the vehicle. Further, 
the brake fluid pressure Pr which is equal to the master cylinder pressure 
Pmc multiplied by the constant .alpha..sub.r0, can be supplied to the 
wheel cylinders 110 and 116 provided for the rear left (RL) wheel and the 
rear right (RR) wheel of the vehicle. Therefore, it is possible that the 
hydraulic brake control apparatus of the above-described embodiment safely 
produce an adequately great braking force on both the front wheels and the 
rear wheels of the vehicle, and the braking force thus produced is 
responsive to the brake operating force Fp. After the step 204 is 
performed, step 204 is performed, step 206 is executed by the ECU 10. 
Step 206 makes a determination as to whether the high-pressure defect in 
the front linear valves 58 and 66 has occurred. More specifically, in the 
step 206, it is detected whether the pressure Pf detected by the Pf sensor 
74 is higher than a sum of the expected supply pressure 
".alpha..sub.f0.Pmc" and a predetermined value ".beta.". 
When the condition at the step 206: Pf&gt;.alpha..sub.f0.Pmc+.beta. is not 
satisfied, it is determined that an appropriate level of the brake fluid 
pressure Pf is supplied to the wheel cylinders 82 and 88 by the front 
linear valves 58 and 66. Thus, when the result at the step 206 is 
negative, the control routine at the present cycle ends. 
On the other hand, when the condition: Pf&gt;.alpha..sub.f0,Pmc+.beta. is 
satisfied, it is determined that the high-pressure defect in the front 
linear valves 58 and 66 has occurred. That is, the detected pressure Pf is 
excessively higher than the expected supply pressure ".alpha..sub.f0.Pmc". 
Thus, when the result at the step 206 is affirmative, it is determined 
that the high-pressure defect in the front linear valves 58 and 66 has 
occurred. In this case, step 208 is executed by the ECU 10 in order to 
carry out a fail-safe function against the high-pressure defect in the 
front linear valves 58 and 66. 
Step 208 sets the FCV 70 at the closed position and sets the pressure hold 
valves 78 and 84 at the closed positions. After the step 208 is performed, 
the supply of the brake fluid pressure from the front linear valves 58 and 
66 to the front hydraulic circuit 72 is inhibited by the FCV 70 which is 
set at the closed position, and the connection passage 76 of the front 
hydraulic circuit 72 and the wheel cylinders 82 and 88 are separated from 
each other by the valves 78 and 84 which are set at the closed positions. 
After the step 208 is performed, step 210 is executed by the ECU 10. 
Step 210 sets the RVCV 96 at the opened position and sets the pressure-down 
valves 90 and 92 at the opened positions. After the step 210 is performed, 
the flow of the brake fluid from the wheel cylinders 82 and 88 into the 
reservoir tank 28 through the RVCV 96 and the valves 90 and 92 is allowed. 
The brake fluid in the wheel cylinders 82 and 88 is set at a high pressure 
upon occurrence of the high-pressure defect in the front linear valves 58 
and 66. Therefore, after the step 210 is performed, the brake fluid 
pressure Pwc in the wheel cylinders 82 and 88 is reduced to a low pressure 
which is approximately equal to the atmospheric pressure. After the step 
210 is performed, step 212, shown in FIG. 2B, is executed by the ECU 10. 
As described above, the check valve 38 allows the flow of the brake fluid 
from the pressure passage 34 to the pressure passage 40 through the bypass 
passage of the MCV 36. If the brake fluid pressure in the front hydraulic 
circuit 72 (or the pressure in the connection passage 76) is lower than 
the brake fluid pressure Pmc in the master cylinder 14 when the pressure 
in the wheel cylinders 82 and 88 is reduced, the brake fluid from the 
master cylinder 14 may flow into the front hydraulic circuit 72 through 
the bypass passage in which the check valve 38 is provided. 
In the above-described steps 208 and 210, the connection passage 76 and the 
wheel cylinders 82 and 88 are separated from each other by the valves 78 
and 84, and the brake fluid from the wheel cylinders 82 and 88 flows into 
the reservoir tank 28 through the RVCV 96 and the valves 90 and 92 so that 
the pressure in the wheel cylinders 82 and 88 is reduced. Therefore, after 
the step 210 is performed, it is possible to safely prevent the flow of 
the brake fluid from the master cylinder 14 into the front hydraulic 
circuit 72 through the bypass passage when the brake fluid from the wheel 
cylinders 82 and 88 flows into the reservoir tank 28 through the RVCV 96 
and the valves 90 and 92. 
As shown in FIG. 2B, step 212 detects whether the elapsed time counted by a 
timer (not shown) exceeds a predetermined time "To". The timer starts 
counting the elapsed time from the time the above step 210 is performed. 
The time "To" is preset to a period of time required for the pressure in 
the wheel cylinders 82 and 88, upon the occurrence of the high-pressure 
defect in the front linear valves 58 and 66, to be reduced to the low 
pressure which is approximately equal to the atmospheric pressure. The 
execution of the step 212 is repeated until the result at the step 212 is 
affirmative. When the result at the step 212 is affirmative, step 214 is 
executed by the ECU 10. 
Step 214 sets the pressure hold valves 78 and 84 at the opened positions. 
After the step 214 is performed, the flow of the brake fluid from the 
connection passage 76 of the front hydraulic circuit 72 into the wheel 
cylinders 82 and 88 through the valves 78 and 84 is allowed. The flow of 
the brake fluid from the wheel cylinders 82 and 88 into the reservoir tank 
28 through the RVCV 96 and the valves 90 and 92 is already allowed at the 
step 210. Therefore, after the step 214 is performed, the high-pressure 
brake fluid in the connection passage 76 flows into the reservoir tank 28 
through the valves 78 and 84, so that the brake fluid pressure in the 
connection passage 76 is rapidly reduced. After the step 214 is performed, 
step 216 is executed by the ECU 10. 
Step 216 detects whether the pressure Pf detected by the Pf sensor 74 after 
the reduction of the brake fluid pressure in the connection passage 76 in 
the step 214 is lower than the master cylinder pressure Pmc detected by 
the Pmc sensor 30. The execution of the step 216 is repeated until the 
result at the step 216 is affirmative. When the result at the step 216 is 
affirmative, step 218 is executed by the ECU 10. 
Step 218 sets the RVCV 96 at the closed position and sets the pressure-down 
valves 90 and 92 at the closed positions. After the step 218 is performed, 
the front hydraulic circuit 72 and the reservoir tank 28 are separated 
from each other by the RVCV 96 and the valves 90 and 92. After the step 
218 is performed, step 220 is executed by the ECU 10. 
Step 220 sets the MCV 36 at the opened position. When the MCV 36 is set at 
the opened position, the flow of the brake fluid between the MCV 36 and 
the wheel cylinders 82 and 88 is allowed. Before the step 216 is 
performed, it is already detected that the brake fluid pressure Pf in the 
connection passage 76 is lower than the master cylinder pressure Pmc in 
the master cylinder 14. Therefore, it is possible to safely prevent the 
counter flow of the brake fluid from the front hydraulic circuit 72 into 
the master cylinder 14 after the step 220 is performed. 
After the step 220 is performed, it is possible to produce a wheel cylinder 
pressure in the wheel cylinders 82 and 88 which is substantially equal to 
the master cylinder pressure Pmc in the master cylinder 14. After the step 
220 is performed, the control routine of FIGS. 2A and 2B at the present 
cycle ends. 
According to the hydraulic brake control apparatus in the embodiment of 
FIGS. 2A and 2B, when the high-pressure defect in the front linear valves 
58 and 66 has occurred, the supply of the brake fluid pressure to the 
wheel cylinders 82 and 88 by the front linear valves 58 and 66 is changed 
to the supply of the brake fluid pressure to the wheel cylinders 82 and 88 
by the master cylinder 14 without producing the counter flow of the brake 
fluid into the master cylinder 14. The pressure reduction device, used by 
the conventional apparatus of the above-mentioned publication, is not used 
by the hydraulic brake control apparatus in the present embodiment. 
Therefore, according to the above-described hydraulic brake control 
apparatus, it is possible to provide the fail-safe function against the 
high-pressure defect in the front linear valves 58 and 66 with a low cost 
and assure the durability of the master cylinder 14. 
In the above embodiment of FIGS. 2A and 2B, before the supply of the brake 
fluid pressure to the wheel cylinders 82 and 88 by the linear valves 58 
and 66 is changed to the supply of the brake fluid pressure to the wheel 
cylinders 82 and 88 by the master cylinder 14, the brake fluid pressure in 
the wheel cylinders 82 and 88 is reduced while the connection passage 76 
and the wheel cylinders 82 and 88 are separated from each other. After the 
brake fluid pressure in the wheel cylinders 82 and 88 is reduced, the 
brake fluid pressure Pf in the connection passage 76 is reduced, and, 
then, the supply of the brake fluid pressure to the wheel cylinders 82 and 
88 by the linear valves 58 and 66 is changed to the supply of the brake 
fluid pressure to the wheel cylinders 82 and 88 by the master cylinder 14. 
According to the above-described embodiment, when the high-pressure defect 
in the front linear valves 58 and 66 has occurred, it is possible to 
safely prevent the counter flow of the high-pressure brake fluid from the 
wheel cylinders 82 and 88 into the master cylinder 14, which is 
detrimental to the durability of the master cylinder 14. 
However, the method of reducing the brake fluid pressure in the wheel 
cylinders 82 and 88 is not limited to the above-described embodiment. 
Alternatively, the brake fluid pressure in the wheel cylinders 82 and 88 
may be reduced without separating the connection passage 76 from the wheel 
cylinders 82 and 88. 
According to the alternative method, the reduction of the brake fluid 
pressure in the wheel cylinders 82 and 88 is stopped when the brake fluid 
pressure Pf detected by the Pf sensor 74 is made equal to the pressure Pmc 
detected by the Pmc sensor 30, and, then, the supply of the brake fluid 
pressure to the wheel cylinders 82 and 88 by the linear valves 58 and 66 
is changed to the supply of the brake fluid pressure to the wheel 
cylinders 82 and 88 by the master cylinder 14. According to the 
alternative method, it is also possible to safely prevent the 
high-pressure brake fluid from the wheel cylinders 82 and 88 from being 
returned back to the master cylinder 14 through the MCV 36. 
FIGS. 3A and 3B show another hydraulic brake control routine performed by 
the hydraulic brake control apparatus embodying the present invention. 
Instead of the control routine of FIGS. 2A and 2B, the control routine of 
FIGS. 3A and 3B is executed by the ECU 10 of the brake system according to 
the present embodiment. The control routine of FIGS. 3A and 3B is executed 
in order to carry out the normal control mode when the high-pressure 
defect in the front linear valves 58 and 66 does not occur, and to carry 
out the above-mentioned function when the high-pressure defect in the 
front linear valves 58 and 66 has occurred. 
In the control routine of FIGS. 2A and 2B, the wheel cylinder pressure Pwc 
in the wheel cylinders 82 and 88 is reduced to a low pressure 
approximately equal to the atmospheric pressure before the supply of the 
brake fluid pressure to the wheel cylinders 82 and 88 by the front linear 
valves 58 and 66 is changed to the supply of the brake fluid pressure to 
the wheel cylinders 82 and 88 by the master cylinder 14. If the wheel 
cylinder pressure Pwc in the wheel cylinders 82 and 88 is reduced to a low 
pressure near the atmospheric pressure, an adequately great braking force 
is hardly produced on the FL and FR wheels by the wheel cylinders 82 and 
88. Therefore, before the supply of the brake fluid pressure to the wheel 
cylinders 82 and 88 by the front linear valves 58 and 66 is changed to the 
supply of the brake fluid pressure to the wheel cylinders 82 and 88 by the 
master cylinder 14, the hydraulic brake control apparatus of the 
embodiment of FIGS. 2A and 2B is temporarily in a condition in which an 
adequately great brake force can be produced on only the RL and RR wheels 
by the wheel cylinders 110 and 116 and an adequately great braking force 
cannot be produced on the FL and FR wheels by the wheel cylinders 82 and 
88. 
Further, in the control routine of FIGS. 2A and 2B, when the supply of the 
brake fluid to the wheel cylinders 82 and 88 is performed by the front 
linear valves 58 and 66, the normal control mode is carried out so that 
the wheel cylinder pressure Pwc in the wheel cylinders 82 and 88 is set at 
a pressure equal to the master cylinder pressure Pmc multiplied by the 
predetermined constant. After the supply of the brake fluid pressure to 
the wheel cylinders 82 and 88 by the front linear valves 58 and 66 is 
changed to the supply of the brake fluid pressure to the wheel cylinders 
82 and 88 by the master cylinder 14 upon occurrence of the high-pressure 
defect in the front linear valves 58 and 66, the wheel cylinder pressure 
Pwc in the wheel cylinders 82 and 88 is set to be equal to the master 
cylinder pressure Pmc by the supply of the brake fluid pressure to the 
wheel cylinders 82 and 88 by the master cylinder 14. The hydraulic brake 
control apparatus of the embodiment of FIGS. 2A and 2B at this time is in 
a condition in which a braking force which is the same as the braking 
force during the normal control mode cannot be produced on the FL and FR 
wheels by the wheel cylinders 82 and 88. 
Accordingly, in the control routine of FIGS. 2A and 2B, the braking force 
produced on the vehicle wheels by the wheel cylinders may be considerably 
reduced before and after the supply of the brake fluid pressure to the 
wheel cylinders 82 and 88 by the front linear valves 58 and 66 is changed 
to the supply of the brake fluid pressure to the wheel cylinders 82 and 88 
by the master cylinder 14 upon occurrence of the high-pressure defect in 
the front linear valves 58 and 66. 
An important function of the hydraulic brake control apparatus in the 
embodiment of FIGS. 3A and 3B is to effectively prevent the 
above-described reduction of the braking force on the vehicle wheels by 
the wheel cylinders in the course of the change from the supply of the 
brake fluid pressure by the front linear valves 58 and 66 to the supply of 
the brake fluid pressure by the master cylinder 14 upon occurrence of the 
high-pressure defect in the front linear valves 58 and 66. The control 
routine of FIGS. 3A and 3B is executed in order to carry out the normal 
control mode when the high-pressure defect in the front linear valves 58 
and 66 does not occur, and to carry out the above-described function when 
the high-pressure defect in the front linear valves 58 and 66 has 
occurred. 
In FIGS. 3A and 3B, the steps which are the same as corresponding steps of 
FIGS. 2A and 2B are indicated by the same reference numerals in 
parentheses which are attached to corresponding reference numerals of 
FIGS. 3A and 3B, and a description thereof will be omitted or simplified. 
As shown in FIG. 3A, the ECU 10 at step 300 detects whether the brake pedal 
12 is depressed by the brake operating force. 
When the result at the step 300 is negative, the subsequent steps are not 
performed and the control routine of FIGS. 3A and 3B at the present cycle 
ends. When the result at the step 300 is affirmative, step 302 is executed 
by the ECU 10. 
Step 302 detects whether an error flag (which will be described later) is 
in an ON state. The error flag is set in the ON state when it is detected 
that the high-pressure defect in the front linear valves 58 and 66 has 
occurred. Otherwise the error flag is set in an OFF state. If it is 
detected at the preceding cycle of the control routine that the 
high-pressure defect in the front linear valves 58 and 66 does not occur, 
the error flag is not in the ON state. When the result at the step 302 is 
negative, step 304 and the subsequent steps are executed by the ECU 10. 
Step 304 sets the MCV 36 at the closed position, sets the FCV 70 at the 
opened position, and sets the RCV 100 at the opened position. This step is 
executed to carry out the normal control mode. Step 306 controls the front 
linear valves 58 and 66 such that the pressure Pf (detected by the Pf 
sensor 74) is supplied by the front linear valves 58 and 66 and the ratio 
of the pressure Pf to the pressure Pmc (detected by the Pmc sensor 30) is 
set at the predetermined constant .alpha..sub.f0. Further, step 306 
controls the rear linear valves 60 and 68 such that the pressure Pr 
(detected by the Pr sensor 98) is supplied by the rear linear valves 60 
and 68 and the ratio of the pressure Pr to the pressure Pmc is set at the 
predetermined constant .alpha..sub.r0. After the step 306 is performed, 
the equations Pf=.alpha..sub.f0.Pmc and Pr=.alpha..sub.r0.Pmc are 
satisfied. 
Step 308 makes a determination as to whether the high-pressure defect in 
the front linear valves 58 and 66 has occurred. More specifically, in the 
step 308, it is detected whether the pressure Pf detected by the Pf sensor 
74 is higher than the sum of the expected supply pressure ".alpha..sub.f0. 
Pmc" and the predetermined value ".beta.". 
When the condition at the step 308: Pf&gt;.alpha..sub.f0.Pmc+.beta. is not 
satisfied, it is determined that an appropriate level of the brake fluid 
pressure Pf is supplied to the wheel cylinders 82 and 88 by the front 
linear valves 58 and 66. At this time, the result at the step 308 is 
negative, and the control routine at the present cycle ends. 
When the condition at the step 308: Pf&gt;.alpha..sub.f0.Pmc+.beta. is 
satisfied, it is determined that the high-pressure defect in the front 
linear valves 58 and 66 has occurred. That is, the detected pressure Pf is 
excessively higher than the expected supply pressure ".alpha..sub.f0.Pmc". 
At this time, the result at the step 308 is affirmative, and step 310 and 
subsequent steps are executed in order to carry out the fail-safe function 
against the high-pressure defect in the front linear valves 58 and 66. 
Step 310 sets the FCV 70 at the closed position and sets the pressure hold 
valves 78 and 84 at the closed positions. After the step 310 is performed, 
the supply of the brake fluid pressure from the front linear valves 58 and 
66 to the front hydraulic circuit 72 is inhibited by the FCV 70 which is 
set at the closed position, and the connection passage 76 of the front 
hydraulic circuit 72 and the wheel cylinders 82 and 88 are separated from 
each other by the valves 78 and 84 which are set at the closed positions. 
Step 312 sets the RVCV 96 at the opened position and sets the pressure-down 
valves 90 and 92 at the opened positions. After the step 312 is performed, 
the flow of the brake fluid from the wheel cylinders 82 and 88 into the 
reservoir tank 28 through the RVCV 96 and the valves 90 and 92 is allowed. 
Therefore, the brake fluid pressure Pwc in the wheel cylinders 82 and 88 
is reduced to a low pressure which is approximately equal to the 
atmospheric pressure. After the step 312 is performed, step 314, shown in 
FIG. 3B, is executed by the ECU 10. 
As shown in FIG. 3B, step 314 controls the rear linear valves 60 and 68 
such that the pressure Pr (detected by the Pr sensor 98) is supplied by 
the rear linear valves 60 and 68 and a ratio of the pressure Pr to the 
pressure Pmc (detected by the Pmc sensor 30) is set at a predetermined 
constant .alpha..sub.r1. The constant .alpha..sub.r1 used by the step 314 
is greater than the constant .alpha..sub.r0 used by the step 306 during 
the normal control mode (.alpha..sub.r0 &lt;.alpha..sub.r1). Therefore, after 
the step 314 is performed, the brake fluid pressure Pwc which is higher 
than the brake fluid pressure Pwc produced during the normal control mode 
can be produced in the rear wheel cylinders 110 and 116 for the RL and RR 
wheels of the vehicle. 
Therefore, according to the embodiment of FIGS. 3A and 3B, the braking 
force greater than the braking force produced during the normal control 
mode can be produced by the rear wheel cylinders 110 and 116 before the 
supply of the brake fluid pressure to the wheel cylinders 82 and 88 by the 
front linear valves 58 and 66 is changed to the supply of the brake fluid 
pressure to the wheel cylinders 82 and 88 by the master cylinder 14 upon 
occurrence of the high-pressure defect in the front linear valves 58 and 
66. It is possible for the embodiment of FIGS. 3A and 3B to effectively 
prevent the above-described reduction of the braking force on the vehicle 
wheels by the wheel cylinders in the course of the reduction of the brake 
fluid pressure Pwc in the wheel cylinders 82 and 88 after the occurrence 
of the high-pressure defect in the front linear valves 58 and 66 is 
detected. After the step 314 is performed, step 316 and the subsequent 
steps are executed by the ECU 10. 
Step 316 detects whether the elapsed time counted by the timer exceeds the 
predetermined time "To". The timer starts counting the elapsed time from 
the time the above step 314 is performed. The time "To" is preset to a 
period of time required for the brake fluid pressure Pwc in the wheel 
cylinders 82 and 88, upon the occurrence of the high-pressure defect in 
the front linear valves 58 and 66, to be reduced to the low pressure which 
is approximately equal to the atmospheric pressure. The execution of the 
step 316 is repeated until the result at the step 316 is affirmative. When 
the result at the step 316 is affirmative, step 318 is executed by the ECU 
10. 
Step 318 sets the pressure hold valves 78 and 84 at the opened positions. 
After the step 318 is performed, the flow of the brake fluid from the 
connection passage 76 of the front hydraulic circuit 72 into the wheel 
cylinders 82 and 88 through the valves 78 and 84 is allowed. The flow of 
the brake fluid from the wheel cylinders 82 and 88 into the reservoir tank 
28 through the RVCV 96 and the valves 90 and 92 is already allowed at the 
step 312. Therefore, after the step 318 is performed, the high-pressure 
brake fluid in the connection passage 76 flows into the reservoir tank 28 
through the valves 78 and 84, so that the brake fluid pressure in the 
connection passage 76 is rapidly reduced. 
Step 320 detects whether the pressure Pf detected by the Pf sensor 74 after 
the reduction of the brake fluid pressure in the connection passage 76 in 
the step 318 is lower than the master cylinder pressure Pmc detected by 
the Pmc sensor 30. The execution of the step 320 is repeated until the 
result at the step 320 is affirmative. When the result at the step 320 is 
affirmative, step 322 is executed by the ECU 10. 
Step 322 sets the RVCV 96 at the closed position and sets the pressure-down 
valves 90 and 92 at the closed positions. After the step 322 is performed, 
the front hydraulic circuit 72 and the reservoir tank 28 are separated 
from each other by the RVCV 96 and the valves 90 and 92. 
Step 324 sets the MCV 36 at the opened position. When the MCV 36 is set at 
the opened position, the flow of the brake fluid between the MCV 36 and 
the wheel cylinders 82 and 88 is allowed. Before the step 324 is 
performed, it is detected that the brake fluid pressure Pf in the 
connection passage 76 is lower than the master cylinder pressure Pmc in 
the master cylinder 14. It is possible to safely prevent the counter flow 
of the brake fluid from the front hydraulic circuit 72 into the master 
cylinder 14 after the step 324 is performed. After the step 324 is 
performed, step 326 is executed by the ECU 10. 
Step 326 controls the rear control valves 60 and 68 such that the pressure 
Pr (detected by the Pr sensor 98) is supplied by the rear linear valves 60 
and 68 and a ratio of the pressure Pr to the pressure Pmc (detected by the 
Pmc sensor 30) is set at a predetermined constant .alpha..sub.r2. The 
constant .alpha..sub.r2 used by the step 326 is greater than the constant 
.alpha..sub.r0 used by the step 306 during the normal control mode and 
smaller than the constant .alpha..sub.r1 used by the step 314 during the 
reduction of the brake fluid pressure Pwc (.alpha..sub.r0 &lt;.alpha..sub.r2 
&lt;.alpha..sub.r1). Therefore, after the step 326 is performed, the brake 
fluid pressure Pwc which is higher than the brake fluid pressure Pwc 
produced during the normal control mode and lower than the brake fluid 
pressure Pwc produced during the reduction of the brake fluid pressure Pwc 
can be produced in the rear wheel cylinders 110 and 116 for the RL and RR 
wheels of the vehicle. 
Therefore, according to the embodiment of FIGS. 3A and 3B, the braking 
force which is greater than the braking force produced during the normal 
control mode and smaller than the braking force produced during the 
reduction of the brake fluid pressure Pwc can be produced by the rear 
wheel cylinders 110 and 116 after the supply of the brake fluid pressure 
to the wheel cylinders 82 and 88 by the front linear valves 58 and 66 is 
changed to the supply of the brake fluid pressure to the wheel cylinders 
82 and 88 by the master cylinder 14 upon occurrence of the high-pressure 
defect in the front linear valves 58 and 66. It is possible for the 
embodiment of FIGS. 3A and 3B to effectively prevent the above-described 
reduction of the braking force on the vehicle wheels by the wheel 
cylinders after the supply of the brake fluid pressure by the front linear 
valves 58 and 66 is changed to the supply of the brake fluid pressure by 
the master cylinder 14 when the occurrence of the high-pressure defect in 
the front linear valves 58 and 66 is detected. After the step 326 is 
performed, step 328 is executed by the ECU 10. 
Step 328 sets the error flag in the ON state, which indicates that the 
high-pressure defect in the front linear valves 58 and 66 has occurred. 
After the step 328 is performed, the control routine of FIGS. 3A and 3B at 
the present cycle ends. 
If the error flag is set in the ON state by the step 328, the result at the 
step 302 during the execution of the control routine at the following 
cycle is affirmative. When the result at the step 302 is affirmative, step 
330 is executed by the ECU 10. 
Step 330 detects whether the pressure Pmc (detected by the Pmc sensor 30) 
is higher than a pressure (Pf-.tau.) and lower than a pressure (Pf+.tau.) 
where Pf is the pressure detected by the Pf sensor 74 and .tau. is a 
predetermined value. The predetermined value .tau. is given by taking into 
account measurement errors and tolerances of the Pmc sensor 30 and the Pf 
sensor 74. When the condition at the step 330: 
(Pf-.tau.).ltoreq.Pmc.ltoreq.(Pf+.tau.) is satisfied, it is determined 
that the brake fluid pressure Pwc in the wheel cylinders 82 and 88 is 
increased to a proper pressure by the supply of the brake fluid pressure 
by the master cylinder 14. At this time, the result at the step 330 is 
affirmative, and step 332 is executed by the ECU 10. 
Step 332 controls the rear control valves 60 and 68 such that the pressure 
Pr (detected by the Pr sensor 98) is supplied by the rear linear valves 60 
and 68 and the ratio of the pressure Pr to the pressure Pmc (detected by 
the Pmc sensor 30) is set at the predetermined constant .alpha..sub.r2. 
The step 332 is similar to the step 326. Therefore, after the step 332 is 
performed, the braking force which is greater than the braking force 
produced during the normal control mode can be produced by the rear wheel 
cylinders 110 and 116 after the supply of the brake fluid pressure to the 
wheel cylinders 82 and 88 by the front linear valves 58 and 66 is changed 
to the supply of the brake fluid pressure to the wheel cylinders 82 and 88 
by the master cylinder 14 upon occurrence of the high-pressure defect in 
the front linear valves 58 and 66. After the step 322 is performed, the 
control routine of FIGS. 3A and 3B at the present cycle ends. 
On the other hand, when the result at the step 330 is negative, it is 
determined that the brake fluid pressure Pwc in the wheel cylinders 82 and 
88 is not increased to a proper pressure by the supply of the brake fluid 
pressure by the master cylinder 14. Therefore, it is determined that 
another defect, other than the high-pressure defect in the front linear 
valves 58, has occurred in the brake system. At this time, step 334 is 
executed by the ECU 10. 
Step 334 sets a system failure flag in an ON state, which indicates that 
another defect in the brake system has occurred. After the step 334 is 
performed, the control routine of FIGS. 3A and 3B at the present cycle 
ends. 
According to the hydraulic brake control apparatus in the embodiment of 
FIGS. 3A and 3B, when the high-pressure defect in the front linear valves 
58 and 66 has occurred, the supply of the brake fluid pressure to the 
wheel cylinders 82 and 88 by the front linear valves 58 and 66 is changed 
to the supply of the brake fluid pressure to the wheel cylinders 82 and 88 
by the master cylinder 14 without producing the counter flow of the brake 
fluid into the master cylinder 14. Therefore, according to the 
above-described hydraulic brake control apparatus, it is possible to 
provide the fail-safe function against the high-pressure defect in the 
front linear valves 58 and 66 and assure the durability of the master 
cylinder 14. 
In addition, according to the hydraulic brake control apparatus in the 
embodiment of FIGS. 3A and 3B, it is possible to effectively prevent the 
above-described reduction of the braking force on the vehicle wheels by 
the wheel cylinders in the course of the change from the supply of the 
brake fluid pressure by the front linear valves 58 and 66 to the supply of 
the brake fluid pressure by the master cylinder 14 upon occurrence of the 
high-pressure defect in the front linear valves 58 and 66. Therefore, the 
above-described hydraulic brake control apparatus enables the brake system 
to maintain an adequately great brake force on the vehicle wheels even 
when the high-pressure defect in the front linear valves 58 and 66 has 
occurred. 
Further, the present invention is not limited to the above-described 
embodiments, and variations and modifications may be made without 
departing from the present invention.