Function

The brake servo unit intensifies the force of the driver’s foot on the brake pedal. The pneumatic brake servo unit was developed in the 60’s, when the bigger cars made a switch from drum brakes to disc brakes at the front axle. There was no more self reinforcement, and the required pedal pressure was too high. In addition, the car industry at that time was particularly inspired by the US–market, where all kinds of auxiliary facilities were used. The first units intensified so strongly that with a little pedal pressure the vehicle attempted a full braking. This has changed, of course.

How it works

If the vehicle is equipped with an Otto engine with a throttle plate, the negative pressure in the intake pipe is usually sufficient to operate the brake servo unit. All other engine types, including the Diesel engine, have to have a vacuum pump. In both cases, while the engine is running, the negative pressure is directed through a one way valve into both chambers of the brake servo unit. As long as the brake is not operated both chambers are connected with each other via channels. There is a movable diaphragm in between the chambers.
If the driver starts braking, the connecting rod moves left, pressing the seal of the double valve against the valve seat, thereby closing the connection between the two chambers. In accordance with the brake pedal force, more or less air at atmospheric pressure gets into the right chamber via an air cleaner (see arrows!), intensifying the brake force. The pressure in the right working chamber is bigger than in the left vacuum chamber. The diameter of the servo unit, still indicated in inches, determines the maximum braking force. For heavier vehicles one might make use of three or four chambers if there is not enough space for one sufficiently large unit. If there is a hydraulic system already present, the support work might be taken over by a substantially smaller hydraulic servo brake.

Non-braking mode

The brake pedal is connected to the brown tappet and the black ring seal via the piston rod. Both the tappet and the ring seal exhibit a small distance to their respective mounting faces. There is negative pressure in both chambers. The outside connection is blocked, because the brown tappet has contact with the black ring seal.

Partly-braking mode

The piston rod is moved leftward. The brown tappet ceases the contact with the black ring seal. The green spring moves it and consequently disconnects the two chambers from each other. The right working chamber is (partly) vented with air at atmospheric pressure. This lasts until the diaphragm with the complete control box is so far left that the brown tappet is connected to the black ring seal again. The increase of pressure in the right chamber depends on the brake pedal force.

Full-braking mode

In the full-braking mode the black ring seal cannot be moved further left. There is no connection with the brown tappet. In the right chamber the full atmospheric pressure is effective.