Reaction mechanism of Brake Pedal


1. General

This mechanism is provided to reduce brake pedal kickback, thereby improving pedal “feel”, by causing only half of the feedback pressure to be applied to the pedal (the other half being absorbed by the booster piston).

2. Operation

The reaction mechanism is shown in the left. The booster push rod, reaction disc and air valve slide inside the valve body. Since the reaction disc is made of soft rubber, it can be regarded as a non-compressible fluid. For this reason, when the booster push rod is pushed to the right, it attempts to compress the reaction disc, but since it cannot, the force is transmitted to the air valve and the valve body. Therefore, the force is transmitted between the air valve and the valve body in proportion to their surface areas. Assume that 100 N (9.8kgf,.21.6 lbf) is applied to the booster push rod, as shown here. Since the ratio of the areas of the air valve and the valve body is 4 to 1, 80 N (7.8kgf; 17.2 lbf) is transmitted to the valve body and 20 N (2.0 kgf,.4.4lbf)to the air valve.

Gap Adjustment of Push Rod

The length of the booster push rod must be adjusted before the brake master cylinder and the brake booster are assembled. This is required so there will be an appropriate gap between the master cylinder piston and the booster push rod after they are reassembled. A SST is used to adjust the gap. In recent models, there are times when a thickness gauge must be used. Be sure to refer to the repair manual.


• When the master cylinder has been replaced and there is an accessory tool in the kit, use the accessory tool to make the adjustment.

• When the label shown in the figure at left is affixed to the booster body, refer repair manual when adjusting the length of the booster push rod.


If the gap is too small, it will cause brake drag. If the gap is too large, it will cause braking delay.

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