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  Hydraulic Brake - Brake Booster

In general, a push rod leads directly from the brake pedal into the brake booster, here with pneumatic reinforcing. There are exceptions though. Since the bulkhead to which both are attached, not always exactly vertical, but often above a little towards the interior is directed, the brake booster may not fit under the (engine) hood.

In this case, purely mechanical deflections are possible, for example towards the middle of the front room. Anyway, it is not always easy to accommodate the relatively large device. Just think about the option of right- hand drive, which has hardly lost any importance lately. In addition, the vehicles are getting heavier, which means more diameter for the brake booster, which incidentally is measured in inches.

Now let's leave aside differences between gasoline-diesel and electric drive for a moment and go out in the picture at the top left of a line with sufficient vacuum. Very important at this point that both chambers are subjected to this vacuum, left the currently larger and right, separated by a diaphragm, the smaller one. We are in driving position.

In this picture you may recognize the sensitive support of the braking force during partial braking. Where the arrows are, atmospheric air flows in through a small air filter and floods the right, still fairly small chamber. There ia a relatively low force on the brake pedal The connection between both chambers is closed and the right chamber remains open to the atmosphere.

The membrane is moved to the left, thus supporting the braking force. By this, the connection between the two chambers opens again. It continues until the pressure of the hydraulics in the master cylinder reaches a certain value corresponding to the pedal position. Then both connections are closed. The full negative pressure is not reached, which is indicated here by a weaker tint than in the next picture.

Now, the pedal has moved its push rod to the left so far that the connection between atmospheric air and right chamber remains open. Thus, the maximum pressure is reached on the right and is maintained even as long as the pedal force is unchanged. This is full braking with maximum support of the pedal force. This depends on the diameter of the brake booster and thus on the diaphragm. Eleven inches represent a maximum here.

Here you can see a possibility to still accommodate the vacuum brake booster, if the space is not sufficient because of a too large diameter. Here are set two smaller diameter amplifiers one after the other, resulting in a total of four chambers. All the chambers are pumped quite empty and then the chambers 1 and 3 from the left partially or completely flooded with atmospheric pressure.

The question remains, why at first pumping all chambers pretty much empty, and then flood two of them? Because you need the reinforcement of the braking force if necessary very quickly and it would take too long to pump one or two chambers then airless. Atmospheric pressure can not be produced as quickly as vice versa.

Yes, the vacuum is the actual source of energy, which increases the braking power. Since the diesel engine has no real throttle in the inlet manifold and now some petrol engines also not, no negative pressure is created there, which could then be forwarded to the brake booster. There are various air pumps and a very important one-way valve usually very near to the brake booster. You can see a design of vacuum pumps at the picture above.

As the eccentric shaft rotates, the slider moves back and forth. Because of the many sliding areas, an oil connection is possible. The oil is removed from the air afterwards. The air enters on the way between the smallest and the largest room and leaves between the largest and the smallest.

Here is a more modern vacuum pump, this time with vanes. These are moved outwards by the rotation and seal accordingly. The air is sucked in at the top through an opening in the side wall of the housing and leaves the pump space below via another one. On the way between inlet and outlet, a pressure builds up, which escapes at the outlet.

It may not occur any leak in the entire system to the brake booster. In a gasoline engine with throttle unregistered secondary air can be sucked, which can bring a mixture preparation properly messed up. Conversely, a clogged air filter can cause unwanted partly braking. Engine and brake booster are not independent from each other.


There are vehicles with internal combustion engine, where the controling system can stop the engine while driving. It is called 'sailing'. If the brake system remains conventional, here an electric vacuum pump is necessary because there is no vacuum available directly from the engine, nor a pump is driven by this. Above you can see such a system.

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