A power booster or power brake unit uses a vacuum to multiply the driver’s pedal effort, and apply that to the master cylinder. This increases the pressures available from the master cylinder.
Units on petrol/gasoline engines use the vacuum produced in the intake manifold.
Vehicles with diesel engines cannot use manifold vacuum so they are fitted with an engine-driven vacuum pump.
The most common booster now operates between the brake and master cylinder. It increases the force that acts on the master cylinder.
Whenever the pedal is depressed, the power brake unit assists the driver. The level of assistance depends on the pressure applied.
Power Brakes
Power brakes are also referred to as 'power assisted brakes' and are designed to use the power from the engines and/or battery to increase the braking power of the vehicle. There are four commonly utilised types of power boosting braking performance. These are:
The majority of cars and light vehicles opt for the use of 'vacuum suspended' units (commonly referred to as Vacuum Boosters), which employ a vacuum assisted power boosting device to provide the additional thrust to the driver's pressure to the brake pedal.
With some hydraulic booster systems, they can tap into the power steering hydraulic circuitry to use the power from the power steering to supplement the actuating pressure to the master cylinder. The electro-hydraulic system uses an electric motor to pressurize the hydraulic system which supplements the pressure to the master cylinder. This allows a braking force to exist in the advent of an engine shutdown or total loss of power.
To test if a vacuum sourced power booster is operational, shut the engine down, then pump the brake pedal until all the power source (such as the vacuum) is exhausted. The pedal should feel hard with a shorter travel. Hold your foot hard on the brake pedal, and start the engine. If the booster is operating correctly, the pedal will depress slightly as the vacuum enters the power booster chamber. This indicates that the booster is receiving a vacuum that will assist in increasing the braking performance. This assumes that the rest of the braking system is functioning correctly according to the manufacturer’s specifications.
In the vacuum type booster system, the pressure on the brake pedal pushes a pushrod connected between between the pistons within the master cylinder.
At this time:
Then:
Source: CDX Global & Wikipedia - en.wikipedia.org
