How vacuum brake booster works?

The vacuum booster operates as follows. In the initial position during factory adjustment, screw 21 achieves a position of the central valve in which the inner part of the valve, pressing against the end of the piston, closes hole B (i.e., the air valve will be closed), and the outer part of the valve moves away from the outer end of the piston (vacuum valve will be open). The article about vacuum brake system construction can help you to understand how it works.

In this position, the air from the cavity 2 through the holes A in the piston 15, the holes in the support disk 1 and the support plate 2 passes into the cavity 1, as a result of which there will be the same vacuum on both sides of the membrane in the cavities 1 and 2. (B)

How vacuum brake booster works
How vacuum brake booster works

When you press the brake pedal, the pusher 17 (a) moves the piston 15 to the left by the amount of the gap B. In this case, the piston moves the valve 4 to the left. But since this force is not enough to overcome the resistance of the spring 7, the support plate 2 does not move. The steel sectors of the support disk 1 bend around the annular rim of the support plate 2 and the gap between the outer ring seat at the end of the piston 15 and the central valve 4 is eliminated, the vacuum valve closes and the cavities 1 and 2 are disconnected.

The initial value of the deflection of the sectors of the support disk is ensured by the structural dimensions of the parts creating, after assembly, the necessary clearance B between the head of the adjusting screw 21 and the support pusher 14. With further movement of the piston and an increase in the deflection of the sectors of the support disk, the central part of the valve 4 moves away from the inner annular end of the piston 15, and the air valve opens.


Consequently, with the vacuum valve closed, the air valve will open. Atmospheric air, passing through the filter 18 and the cavity inside the piston 15, through the holes A and B will enter the cavity 2 of the amplifier. The pressure behind the membrane will be greater than in the cavity 1, which is constantly connected to the inlet pipe of the engine. The membrane through the support disk 1, the support plate 2 and the lock washer 5 transfers additional force to the rod 6, which is combined with the force transmitted to the same rod from the brake pedal through the fork 19, pusher 17, support 14 and the adjusting screw 21.

The additional force generated by the booster will be proportional to the force applied to the brake pedal. This proportionality is ensured by a certain ratio of the active areas of the membrane located on both sides of the circle of contact between the sectors of the support disk 1 and the support plate 2.


Suppose the driver, pressing the brake pedal, locked the pedal in some position. Therefore, the piston 15 and the membrane 12 with the support disk 1 will occupy a fixed position. A certain pressure will be established in the cavity 2, but since the area of ​​the membrane external to the circumference of the support plate 2 is larger than the inside, the force on it will be higher. The sectors of the support disk will rotate relative to their points of contact with the support plate 2; while the central valve is pressed against the inner annular end of the piston and both valves – air and vacuum – will be closed, and the action of the amplifier will remain constant.

If you lightly depress the pedal, this condition will come faster, as the sectors of the support disk will bend slightly and quickly close the air valve. In this case, a little air will enter cavity 2, and the additional force generated by the amplifier will be negligible.

When braking, the brake pedal, returning to its original position, pulls the pusher 17. The latter returns the piston to its original position. Deflection of the support disk stops. The Central valve under the action of the spring 3 is pressed against the inner annular end of the piston (air valve is closed). A gap will appear between the outer annular end of the piston and the central valve (the vacuum valve will open). In cavities 1 and 2 will be the same discharge. Under the action of the spring of the brake pedal and using the return spring 7, the support plate together with the support disk and the membrane will take its original position.


The technical condition of the brake system has a decisive influence on driving safety. Braking performance should be up to standard. Braking distance and allowable deceleration of the car (initial braking speed of 40 km / h.

Car Braking distance, m, no more Slowdown m / s2
Light 16,2 5,2
Truck 23 4
Road train 25 4

The parking brake must hold the car regardless of its load on the ascent or descent with a slope of 31% for trucks and 23% for cars.

The unsatisfactory condition of the control mechanisms is one of the main causes of road traffic accidents arising from technical malfunctions of the car.

Author: delfi

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