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The force on the hinge C is in part due to the piston and in part due to the driver's pressure on the pedal. It will be noticed that the driver's foot is the fulcrum of this system and it is due to this fact that the driver at all times retains a direct and sensitive control over the braking effort applied by the mechanism. Any tendency on the part of the piston to exert an increased force on the lever A necessarily re-acts on the pedal. If the driver merely exerts a light pressure on the pedal, the position of H is such that any tendency for the piston to move far along the cylinder will cause the lever A to swing about H in such a way as to close the valve G, thereby restricting the vacuum in the cylinder to an amount proportional to the force exerted on the pedal. It has already been explained that the net effect of the piston is to treble the force exerted by the driver on the pedal over the first half of the range. The diagram on page 21 illustrates this graphically and also shows the leverage effect obtained with greater pedal pressures.

The fact that the power mechanism is in working order can be very simply tested as follows.