In order to decrease the speed of a running vehicle and bring it to a stop, it is necessary to generate force to slow the rotation of the tires. When the driver operates the brake pedal, the brake device generates the force (road surface counterforce) that works to stop the tires and the force (inertia) working to keep the vehicle going is absorbed, thus stopping the vehicle. In other words, the energy (kinetic energy) of the tires working to rotate is converted into the heat of friction (thermal energy) by operating the brakes which works to stop the rotation of the tires. The vehicle must not only stop but must be able to stop in a way that reflects the intent of the driver. For example, the brakes must decrease the vehicle speed at the desired rate of the deceleration and stop in a relatively stable manner in a comparatively short distance during emergency braking. The main devices making up this stop function are the brake system such as the brake pedal, and the tires.

Brake System

There are two types of brake systems. The main brake system used when the vehicle is running is the foot brake system. There are drum brake and disc brake that are most commonly operated with hydraulic pressure. The parking brake system is used when leaving the vehicle is parked. The parking brake system operates the rear wheel brakes via wires or the like so that the vehicle does not move.

ABS (Anti-lock Brake System)

An ABS is a brake control unit that uses a computer control to automatically prevent the tires from locking due to emergency braking. This system raises the vehicle stability further and shortens the braking distance. Therefore, the tires do not lock up and the steering wheel can be steered even when the brake is depressed suddenly. The vehicle is kept in control, and can stop safely.

ABS with EBD

The “EBD” in the ABS with EBD is the abbreviation for Electronic Brake force Distribution or the Electronic Braking force Distribution control for the ABS. In addition to the conventional ABS function, braking force appropriate to the condition of the vehicle is distributed among the front and rear wheels and left and right wheels using the ABS brake hydraulic control unit.

BA (Brake Assist)

The BA is a system that assists brake

operation when drivers cannot apply

enough force on the brake pedal. Sudden

pressure applied to the brake pedal

is judged as an emergency stop, and a

larger amount of braking force is automatically

generated.

The brake system consists of the following components.

1. Brake pedal

2. Brake booster

3. Master cylinder

4. Proportioning valve (P valve)

5. Foot brake

(1) Disc brake

(2) Drum brake

6. Parking brake

The master cylinder is a device that converts the operation force applied by the brake pedal into hydraulic pressure. Currently, the tandem master cylinder, which includes two pistons, generate hydraulic pressure in two-systems brake line. The hydraulic pressure is then applied to the disc brake calipers or the wheel cylinders of the drum brakes. The reservoir serves to absorb changes in the brake fluid volume caused by changes in fluid temperature. Also, it has a separator inside that divides the tank into front and rear parts as shown in the left. The twopart design of the tank ensures that if one circuit fails due to fluid leakage, the other circuit will still be available to stop the vehicle. The fluid level sensor detects when the fluid level in the reservoir tank falls below the minimum level and then uses the brake system warning light to warn the driver.

2. Construction

The master cylinder consists of the following components.

(1) No.1 piston

(2) No.1 return spring

(3) No.2 piston

(4) No.2 return spring

(5) Rubber piston cups

(6) Reservoir tank

(7) Fluid level sensor

3. Principles

When the brake pedal is depressed, the master cylinder converts this force into hydraulic pressure. Brake pedal operation is based on the principle of the lever, and converting a small pedal force into a large force acting on the master cylinder. Based on Pascal’s law, the hydraulic force generated in the master cylinder is transmitted via brake line to individual wheel cylinders. It acts on brake linings and disc brake pads to generate a braking force. According to Pascal’s law, externally applied pressure upon a confined fluid is transmitted uniformly in all directions. Applying this principle to a hydraulic circuit in a brake system, the pressure generated in the master cylinder is transmitted equally to all wheel cylinders. The braking force varies, as shown in the left, depending on the diameter of the wheel cylinders. If a vehicle design requires a larger braking force at the front wheels, for example, the designer will specify larger wheel cylinders for the front.

4. Types of brake lines

If the brake line is cracked and the brake fluid leaks out, the brakes will no longer work. For this reason, the brake hydraulics are divided into two-systems brake line. The hydraulic pressure sent to the two systems from the master cylinder is transmitted to the disc brake calipers or wheel cylinders. The brake line layout differs between FR vehicles and FF vehicles. In FR vehicles the brake lines are divided in to a front wheel system and rear wheel system, but in FF vehicles diagonal piping is used. Because the load applied to the front in FF vehicles is large, a higher braking force is used for the front wheels than for the rear wheels. For this reason, if the same brake line systems used for FR vehicles are used in FF vehicles, the braking force will be too weak if the front wheel braking system fails, so a diagonal pipe line system for the front right wheel and rear left wheel and one for the front left wheel and rear right wheel are used so that if one system fails, the other system will maintain a certain degree of braking force.

When the brake pedal is depressed, the force is transmitted via the push rod to the master cylinder where the piston is pushed.

The force of the hydraulic pressure generated inside the master cylinder is transmitted via the brake lines to the each wheel cylinder.

1. Normal operation

(1) When the brakes are not applied.

The piston cups of No.1 and No.2 piston are positioned between the inlet port and the compensating port, providing a passage between the master cylinder and the reservoir tank. No.2 piston is pushed to the right by force of No.2 return spring, but prevented from going any further by a stopper bolt.

(2) When the brake pedal is

depressed

No.1 piston moves to the left and the piston cup seals the compensating port to block the passage between the cylinder and the reservoir tank. As the piston is pushed farther, it increases the hydraulic pressure inside the master cylinder. This pressure acts on the rear wheel cylinders. Since the same hydraulic pressure also pushes No.2 piston, No.2 piston operates in exactly the same way as No.1 piston, and acts on the front wheel cylinders.

(3) When the brake pedal is released.

The pistons are returned to their original position by hydraulic pressure and the force of the return springs. However, because the brake fluid does not return from the wheel cylinder immediately, the hydraulic pressure inside the master cylinder momentarily drops (a vacuum develops). As a result, the brake fluid inside the reservoir tank flows into the master cylinder via the inlet port, through many orifices provided at the tip of the piston, and around the periphery of the piston cup. After the piston has returned to its original position, the brake fluid that gradually returns from the wheel cylinder to the master cylinder flows into the reservoir tank through the compensating ports. The compensating port also absorbs changes in brake fluid volume that could occur inside the cylinder due to temperature changes. This prevents the hydraulic pressure from rising when the brakes are not being used.

2. If fluid leaks in one of the systems.

(1) Fluid leakage in rear side When the brake pedal is depressed, No.1 piston moves to the left but does not create hydraulic pressure in the rear side. No.1 piston therefore compresses the return spring, contacts No.2 piston, and pushes it No.2 piston increases hydraulic pressure in the front end of the master cylinder, which allows two of the brakes to be operated from the front of the master cylinder.

(2) Fluid leakage in front side

Since hydraulic pressure is not generated in the front side, No.2 piston advances until it contacts the wall at the far end of the master cylinder. When No.1 piston is pushed farther to the left from this position, hydraulic pressure increases in the rear side of the master cylinder, which allows two of the brakes to be operated from the rear of the master cylinder.

1. General

The brake booster is a device that utilizes the difference between the engine vacuum and the atmospheric pressure to generate a strong force (power boost) that is proportional to the pedal depression force to operate the brakes. The brake booster uses the vacuum generated in the intake manifold (vacuum pump in the case of diesel engines).

2. Construction

The brake booster consists of the following components.

(1) Valve operation rod

(2) Push rod

(3) Booster piston

(4) Booster body

(5) Diaphragm

(6) Diaphragm spring

(7) Valve body

(8) Reaction disc

(9) Air cleaner

(10)Body seal

(11)Variable pressure chamber

(12)Constant pressure chamber

(13)Check valve

A tandem brake booster is a device that has two vacuum chambers positioned in series and obtains a large power boost without having to increase the piston’s size.

1. Brakes not applied

The air valve is connected to the valve operating rod, and pulled to the right by the air valve return spring. The control valve is pushed to the left by the control valve spring. This causes the air valve to contact the control valve. Therefore, the atmospheric air that passes through the air cleaner element is prevented from entering the variable pressure chamber. The valve body’s vacuum valve is separated from the control valve in this condition, providing an opening between passage A and passage B. Since there is always a vacuum in the constant pressure chamber, there is also a vacuum in the variable pressure chamber at this time. As a result, the piston is pushed to the right by the diaphragm spring.

2. Brakes applied

When the brake pedal is depressed, the valve operating rod pushes the air valve, causing it to move to the left. The control valve, pushed against the air valve by the control valve spring, also moves to the left until it contacts the vacuum valve. This blocks off the opening between passage A and passage B. As the air valve moves further to the left, it moves away from the control valve. This allows atmospheric air to enter the variable pressure chamber through passage B (after passing through the air cleaner element). The difference in pressure between the constant pressure chamber and the variable pressure chamber causes the piston to move to the left. This, in turn, causes the reaction disc to move the booster push rod to the left and increase the braking force.

3. Holding state

If the brake pedal is depressed halfway, the valve operating rod and the air valve stop moving but the piston continues to move to the left due to the difference in pressure. The control valve is kept in contact with the vacuum valve by the control valve spring, but moves along with the piston. Since the control valve moves to the left and contacts the air valve, atmospheric air is prevented from entering the variable pressure chamber, so the pressure in the variable pressure chamber stabilizes. As a result, there is a constant difference in pressure between the constant pressure chamber and the variable pressure chamber. Therefore, the piston stops moving and maintains the present braking force.

4. Maximum boost

If the brake pedal is depressed all the way down, the air valve will move completely away from the control valve. In this condition, the variable pressure chamber is filled entirely with atmospheric air, and the difference in pressure between the constant pressure chamber and the variable pressure chamber is maximized. This causes the maximum boosting effect to act on the piston. Even if additional force is thereafter applied to the brake pedal, the boosting effect on the piston will remain unchanged, and the additional force will be applied only to the booster push rod and transmitted as is to the master cylinder.

5. Non-vacuum condition

If a vacuum fails to be applied to the brake booster for any reason, there will be no difference in pressure between the constant pressure chamber and the variable pressure chamber (as both will be filled with atmospheric air). When the brake booster is in the “off” position, the piston is returned to the right by the diaphragm spring. Nevertheless, when the brake pedal is depressed, the valve operating rod advances to the left and pushes the air valve, reaction disc and booster push rod. This causes the master cylinder piston to apply braking force to the brake. At the same time, the air valve pushes the valve stopper key which is inserted into the valve body. Therefore, the piston also overcomes the diaphragm spring and moves to the left. Accordingly, the brakes remain functional even when there is no vacuum applied to the brake booster. However, since the brake booster is not operating, the brake pedal will feel “heavy”.

1. General

This mechanism is provided to reduce brake pedal kickback, thereby improving pedal “feel”, by causing only half of the feedback pressure to be applied to the pedal (the other half being absorbed by the booster piston).

2. Operation

The reaction mechanism is shown in the left. The booster push rod, reaction disc and air valve slide inside the valve body. Since the reaction disc is made of soft rubber, it can be regarded as a non-compressible fluid. For this reason, when the booster push rod is pushed to the right, it attempts to compress the reaction disc, but since it cannot, the force is transmitted to the air valve and the valve body. Therefore, the force is transmitted between the air valve and the valve body in proportion to their surface areas. Assume that 100 N (9.8kgf,.21.6 lbf) is applied to the booster push rod, as shown here. Since the ratio of the areas of the air valve and the valve body is 4 to 1, 80 N (7.8kgf; 17.2 lbf) is transmitted to the valve body and 20 N (2.0 kgf,.4.4lbf)to the air valve.

Gap Adjustment of Push Rod

The length of the booster push rod must be adjusted before the brake master cylinder and the brake booster are assembled. This is required so there will be an appropriate gap between the master cylinder piston and the booster push rod after they are reassembled. A SST is used to adjust the gap. In recent models, there are times when a thickness gauge must be used. Be sure to refer to the repair manual.

HINT:

• When the master cylinder has been replaced and there is an accessory tool in the kit, use the accessory tool to make the adjustment.

• When the label shown in the figure at left is affixed to the booster body, refer repair manual when adjusting the length of the booster push rod.

SERVICE HINT:

If the gap is too small, it will cause brake drag. If the gap is too large, it will cause braking delay.

The brake booster utilizes the difference between the engine vacuum and the atmospheric pressure to generate a power boost. Therefore, the brake booster function can be checked by conducting the following inspection.

1. Airtightness function check

Generating a power boost requires that the vacuum inside the brake booster be maintained, that the constant pressure chamber and variable pressure chamber be completely closed off by the vacuum valve, and that air must flow from the air valve. (1) Stop the engine after running it for 1 to 2 minutes. Vacuum will be allowed into the brake booster. (2) Depress the brake pedal several times. When doing this, if the pedal position is higher 2nd or 3rd time than it was the 1st time, the check valve or vacuum valve is closed, the air valve is open, and air is being let in. From this it can be determined that the airtightness of each valve is normal.

2. Operation check

If the engine is started while there is no vacuum in the brake booster, the vacuum valve is closed, and the air valve is open, vacuum will be allowed into the constant pressure chamber. The brake pedal condition at this time can be used to check the power boost operation.

(1) With the engine stopped, depress the brake pedal several times. Air will be allowed into the constant pressure chamber.

(2) Start the engine with the brake pedal depressed. Vacuum will be generated and a pressure difference will be generated between the constant pressure chamber and the variable pressure chamber. If the brake pedal sinks down a little bit at this time, it can be determined that a normal power boost has been generated. 3. Load airtightness function check

If the engine is turned OFF with the brake pedal depressed, the pedal condition can be used to check for vacuum leaks from the constant pressure chamber. (1) Depress the brake pedal while the engine is running. (2) Turn OFF the engine with the brake pedal depressed. In the hold state, the pressure difference between the constant pressure chamber and the variable pressure chamber will be held constant. Therefore, if there is no change in the brake pedal height while continuing to hold it for 30 sec, then it can be determined that the check valve and vacuum valve are closed normally and that there are no problems with the constant pressure chamber.

1. General

The proportioning valve (P valve) is placed between the brake line master cylinder and the rear wheelÅfs wheel cylinder. This device obtains proper braking force to shorten the braking distance by approaching the ideal front and rear wheel braking force distribution to prevent the rear wheels from early lock up during emergency braking (when the load transfers to the front), etc. When the distribution is like that shown by (a), the braking force becomes large causing the rear wheel braking force to become too much larger than the ideal curve, which makes it easy for the rear wheels to lock up and destabilize the vehicle. In addition, when the distribution is like that shown by (b), the overall braking force becomes small, which allows the front wheels to lock up easily and cause loss of steering control.

2. Construction

The P valve consists of the following components.

(1) Valve body

(2) Piston

(3) Valve seal

(4) Compression spring

(5) Cylinder cup

(1/1)

The hydraulic pressure generated by the master cylinder acts on the front and rear brakes. The rear brakes are controlled so that the hydraulic pressure is kept the same as that of the master cylinder up to the split point and then is made lower than that of the master cylinder after the split point. The P valve operation condition is shown below.

1. Operation up to the split point

The spring force pushes the piston to the right. The hydraulic pressure from the master cylinder passes through the gap between the piston and the cylinder cup to apply an equal force to the front and rear wheel cylinders. At this time, a force works to move the piston to the left utilizing the difference in the pressure reception surface area, but it cannot overcome the spring force, so it does not move.

2. Operation of the split point

When the hydraulic pressure applied to the rear wheel cylinder increases, the pressure pushing the piston to the left overcomes the force of the spring causing the piston to move to the left and close the fluid circuit.

3. Operation after the split point

When the hydraulic pressure from the master cylinder increases even further, this increase in pressure pushes the piston to the right to open the fluid circuit. When this happens, the hydraulic pressure to the rear wheel cylinder begins to rise and the pressure pushing the piston to the left begins to increase, so before the hydraulic pressure to the rear wheel cylinder rises completely, the piston moves to the left and closes the fluid circuit. This valve operation is repeated to keep the hydraulic pressure on the rear wheel side from increasing more than that on the front wheel side.

4. Operation when the pedal is released

When the hydraulic pressure from the master cylinder decreases, the fluid on the rear wheel cylinder side passes through the outside of the cylinder cup and returns to the master cylinder side.

1. Dual P Valve

The dual P valve is used in diagonal brake piping in FF vehicles. Basically, it may be considered as a pair of P valves operating side by side. Each of the two P valves operates in the same manner as an ordinary P valve.

2. Proportioning & Bypass Valve (P & BV)

The P & BV plays two roles. Firstly, it acts as an ordinary P valve. In addition, if the hydraulic circuit for the front brakes fails for any reason, it deactivates the P valve function. (Even if the master cylinder hydraulic pressure increases, the same pressure is transmitted to the rear wheels.)

3. Load Sensing Proportioning Valve (LSPV)

The LSPV is a device that is basically the same as the P valve, but it can adjust the P valve split point in response to the load applied to the rear tires. The LSPV prevents the rear brakes from over braking, locking up, and slipping, and also makes it possible to obtain a large braking force when the rear load is large. This is widely used in vehicles such as trucks, which the load balance applied to front and rear wheels change greatly when the vehicle is loaded and when it is unloaded. The load is detected by a load sensing spring located between the rear axle housing and the frame (or body). The split point can be adjusted by adjusting the spring strength. Sometimes a dual LSPV is used for diagonal piping in FF vehicles.

The EBD control utilizes ABS, helping realize the proper brake force distribution between front and rear wheels in accordance with the driving conditions. In addition, during cornering braking, it also controls the brake forces of right and left wheels, helping to maintain the vehicle stability.

(1) Front/Rear wheels brake force distribution

If the brakes are applied while the vehicle is moving straight forward, the transfer of the load reduces the load that is applied to the rear wheels. The Skid Control ECU determines this condition by way of the signals from the speed sensors, and controls the ABS actuator in order to optimally control the distribution to the brake force to the rear wheels. For example, the amount of the load that is applied to the rear wheels during braking varies whether or not the vehicle is carrying a load. The amount of the load that is applied to the rear wheels also varies in accordance with the extent of the deceleration. Thus, the distribution of the brake force to the rear is optimally controlled in order to effectively utilize the braking force of the rear wheels under these conditions.

(2) Right/Left wheels brake force distribution (During cornering braking)

When the brakes are applied while the vehicle is cornering, the load that is applied to the inner wheel decreases. The Skid Control ECU determines this condition by way of the signals from the speed sensors, and controls the ABS actuator in order to optimally control the distribution of the brake force to the inner wheel.