A tandem brake booster is a device that has two vacuum chambers positioned in series and obtains a large power boost without having to increase the piston’s size.
1. Brakes not applied
The air valve is connected to the valve operating rod, and pulled to the right by the air valve return spring. The control valve is pushed to the left by the control valve spring. This causes the air valve to contact the control valve. Therefore, the atmospheric air that passes through the air cleaner element is prevented from entering the variable pressure chamber. The valve body’s vacuum valve is separated from the control valve in this condition, providing an opening between passage A and passage B. Since there is always a vacuum in the constant pressure chamber, there is also a vacuum in the variable pressure chamber at this time. As a result, the piston is pushed to the right by the diaphragm spring.
2. Brakes applied
When the brake pedal is depressed, the valve operating rod pushes the air valve, causing it to move to the left. The control valve, pushed against the air valve by the control valve spring, also moves to the left until it contacts the vacuum valve. This blocks off the opening between passage A and passage B. As the air valve moves further to the left, it moves away from the control valve. This allows atmospheric air to enter the variable pressure chamber through passage B (after passing through the air cleaner element). The difference in pressure between the constant pressure chamber and the variable pressure chamber causes the piston to move to the left. This, in turn, causes the reaction disc to move the booster push rod to the left and increase the braking force.
3. Holding state
If the brake pedal is depressed halfway, the valve operating rod and the air valve stop moving but the piston continues to move to the left due to the difference in pressure. The control valve is kept in contact with the vacuum valve by the control valve spring, but moves along with the piston. Since the control valve moves to the left and contacts the air valve, atmospheric air is prevented from entering the variable pressure chamber, so the pressure in the variable pressure chamber stabilizes. As a result, there is a constant difference in pressure between the constant pressure chamber and the variable pressure chamber. Therefore, the piston stops moving and maintains the present braking force.
4. Maximum boost
If the brake pedal is depressed all the way down, the air valve will move completely away from the control valve. In this condition, the variable pressure chamber is filled entirely with atmospheric air, and the difference in pressure between the constant pressure chamber and the variable pressure chamber is maximized. This causes the maximum boosting effect to act on the piston. Even if additional force is thereafter applied to the brake pedal, the boosting effect on the piston will remain unchanged, and the additional force will be applied only to the booster push rod and transmitted as is to the master cylinder.
5. Non-vacuum condition
If a vacuum fails to be applied to the brake booster for any reason, there will be no difference in pressure between the constant pressure chamber and the variable pressure chamber (as both will be filled with atmospheric air). When the brake booster is in the “off” position, the piston is returned to the right by the diaphragm spring. Nevertheless, when the brake pedal is depressed, the valve operating rod advances to the left and pushes the air valve, reaction disc and booster push rod. This causes the master cylinder piston to apply braking force to the brake. At the same time, the air valve pushes the valve stopper key which is inserted into the valve body. Therefore, the piston also overcomes the diaphragm spring and moves to the left. Accordingly, the brakes remain functional even when there is no vacuum applied to the brake booster. However, since the brake booster is not operating, the brake pedal will feel “heavy”.