Power assisted brakes use either hydraulic or vacuum pressure to reduce the amount of force needed to press the brake pedal. Hydraulic systems use pressure from the power steering pump or an air compressor, while vacuum systems use pressure from the intake manifold. Both types work by creating pressure on one side of a booster piston to multiply the force applied through the brake pedal to the master cylinder. When the pedal is pressed, it closes off the pressure source and applies hydraulic pressure to the brakes. Releasing the pedal vents the booster pressure and allows the pedal to return freely. Servo brakes provide additional braking power needed for heavier vehicles through mechanical, hydraulic, or vacuum assisted systems.

1. Power Assisted Brakes

2. Power Brakes
•Power Brakes assists in braking when the pedal is pressed.
Hydraulic booster hydraulic pressure is applied by power-steering pump
Vacuum Booster works with the vacuum from the vehicle’s manifold.
To Check press on the brake pedal
and start the vehicle, if working
properly the pedal should go down.

3. CONTENTS
• POWER ASSISTED BRAKES
–Air assisted hydraulic brakes
–Vaccum assisted hydraulic brakes
• SERVO BRAKES
– Mechanical servo mechanism
– Hydraulic servo brakes
– Vaccum servo brakes

4. Air assisted hydraulic brakes

6. Air assisted hydraulic brakes
1. In this type, compressed air is converted into hydraulic pressure.
2. It consists of following parts:
– AIR COMPRESSOR
– AIR TANK
– AIR PRESSURE REGULATOR
– FOOT CONTROL VALVE
– LEVER TYPE SERVO
3. The air compressor delivers compressed air to air tank that is connected to
lever type servo.
4. When the driver presses the pedal the input rod moves towards right.
5. The lower arm helps to close the exhaust valve but opens the inlet valve.
6. Compressed air flows from the air tank into air chamber.

7. 7. The pressure, forces the piston to move right, so
the effort being transferred to master cylinder.
8. The force on the master cylinder piston creates
hydraulic pressure for the application of brakes.
9. On release of pedal, pressure movement of lower
arm opens the exhaust valve and inlet valve
remains closed.
10.The entire air pressure in the chamber is released
into the atmosphere.

8. Vaccum assisted hydraulic brakes

11. Vaccum assisted hydraulic brakes
1. In this type, Vaccum system assists in the operation of hydraulic
brake.
2. The main parts of the system are:
– VACCUM BOOSTER
– VACCUM TANK
3. Here the Vaccum booster helps operating the master cylinder.
4. Vaccum booster has a cylinder. Inside the cylinder a piston
operates a control valve to admit and stop the engine Vaccum
and atmospheric pressure.
5. The atmospheric valve remains closed when the brakes are not
applied.

12. Working
1. When the brake pedal is depressed the atmospheric valve
and Vaccum valve also opens.
2. Now the piston moves by Vaccum created in the Vaccum
cylinder.
3. The piston is connected to the control arm.
4. The open end of the control arm is connected to pushrod of
the master cylinder.
5. Which it turns pushes the piston of the master cylinder.
6. The brake oil pushed to all wheel cylinders under pressure
and the brakes are applied.

13. Servo brakes
Note:- Servo brakes or Power Brakes

14. Servo mechanism
• Any mechanism which adds to the drivers
effort in applying the brakes is called a servo
mechanism.
• Although that effort remains a considerable
part of the total braking effort required.

15. Why servo is used in automobile
• As the weight of vehicle goes on increasing, more
braking effort is needed to stop the vehicle.
• When a limit is reached above, which is not possible
for an ordinary driver to apply the effort needed
conveniently.
• This limit is normally reached at about 3 times the
weight of the vehicle.
• To solve this problem by the help of the servo action.

16. Types of servo brakes
– Mechanical servo mechanism
– Hydraulic servo brakes
– Vaccum servo brakes

18. HYDRAULIC SERVO BRAKES

20. • when extra oil under pressure supplied by a pump
driven by the engine or some part of the
transmission system is used to assist the braking
effort, this system is called hydraulic servo brakes.
HYDRAULIC SERVO BRAKES

21. Construction
1. It consists of pump which forces the fluid from the tank.
2. Through the servo unit, oil back to the tank until the brake pedal is
pressed.
3. The inlet pipe is connected at A and the outlet at B to the servo unit.
4. The servo unit consists of a master cylinder with piston which is provided
with pipes for brakes.
5. A reservoir is connected to master cylinder for providing brake fluid.
6. A plunger is provided in the right side portion of the unit, which is pivoted
with the brake pedal rod.
7. A space S is provided between the master cylinder, piston and plunger for
building up the fluid pressure.

22. Working
• when the brake pedal is at off position then the plunger rests at its right
extreme position there by flowing the fluid through space S from inlet A to
the outlet B of the unit.
• when the brake pedal is pressed it results cut-off, the fluid at outlet B.
• Therefore, the pressure build up in the space S pushes the master cylinder.
• The piston moves towards left there by applying the brakes.
• When the pedal is released the pressure of fluid in space S is exerted on
the left side of the plunger, there by pushing the plunger toward right and
opening the passage at B.
• When the vehicle is at rest ,the servo system is not operative and the
compete unit functions just as ordinary hydraulic braking system.

23. VACCUM SERVO BRAKES

24. History
• Albert Dewandre (Liege, Belgium), an engineer and business
owner, was the inventor of servo-brake or brake booster
system in 1927.
• It is a brake boosting system that uses the depression caused
by the suction in the intake manifold of an internal
combustion engine to reduce the pressure on the brake
pedal.
• The advantage of the Dewandre system is two fold:
1. A softer push on the brake pedal,
2. But also a notably shorter braking distance.
• His invention was manufactured and sold through the Robert
Bosch Company.

25. VACCUM SERVO BRAKES
• When the Vaccum is obtained from the
manifold of the engine or a separate driven
exhauster used to assist the braking effort.
• This system is called Vaccum servo brakes.

27. construction
1. The system consists of a Vaccum reservoir connected through a non-return valve
to the inlet manifold of the engine.
2. Two connections from Vaccum reservoir, one on each side of the piston of the
servo cylinder is provided.
3. On left side the connections are through the control unit where as the right side
is connected directly.
4. The piston of the servo cylinder is further connected to the piston of the booster
cylinder.
5. The control unit consists of a piston to which two valves are attached.
6. The lower valve controls the connection between the reservoir and the right side
of the servo cylinder piston.
7. The upper valve controls the connection between the atmosphere and the left
side of servo cylinder piston.
8. The other side of the piston of the control unit is actuated by the pedal effort
through a master cylinder.

28. working
1. When the brake pedal is at off position, then the lower valve is opened and the
upper valve is closed.
2. Under this position the air from the atmosphere is disconnected and the Vaccum
from reservoir is created on the both sides of the piston of the servo cylinder.
3. When the brake pedal is pressed the brake fluid pushes the piston in the control
unit.
4. This Acton closes the lower valve and opens the upper valve of the control unit.
5. Thereby exposing the left side of the servo cylinder piston to atmospheric
pressure and acting the Vaccum on the right side.
6. This action of the Vaccum in the right side of the servo piston moves it to the
right.
7. There by utilizing this movement through the mechanical or hydraulic means it
the wheel cylinders and applying the brakes.