1. Field of the Invention
This invention relates to a pneumatic-hydraulic brake booster or air-over hydraulic brake booster for a vehicle.
2. Brief Description of the Prior Art
In a conventional pneumatic-hydraulic brake booster, an internal space of a cylindrical casing is divided into two chambers by a power piston which is slidably fitted to the cylindrical casing. In operation, compressed air is supplied to the one chamber. An output rod is fixed to the power piston, and it extends across the other chamber into a hydraulic cylinder body of a master cylinder part.
The compressed air is supplied to the one chamber directly from a brake valve or through a relay valve from the brake valve. In the former case, only one conduit is connected between the brake booster and the brake valve. The brake system is simple. However, in the latter case, one conduit is connected between the brake valve and the relay valve fixed to a part of the chassis of the vehicle, and another conduit should be connected between the relay valve and the brake booster. The brake system is so complicated. More parts are necessary. Manufacturing efficiency or productivity is low.
When the distance between the brake valve and the brake booster is long, without a relay valve the braking operation lags behind the tread of the brake pedal. The retard can be reduced with the relay valve arranged near the brake booster. However, the conventional brake booster has the above described defects.
The same as the assignee of this application filed the pneumatic-hydraulic brake booster (copending application U.S. Ser. No. 634624) to remove the above-described defects. It includes a master cylinder part; a pneumatic cylinder part combined with said master cylinder part, including a power piston slidably fitted to a cylindrical casing, and an output rod fixed to said power piston, extending to said master cylinder part; a partition wall body fitted to said cylindrical casing; and a relay valve part being arranged at the opposite side of said partition wall body to said power piston in said cylindrical casing, for supplying compressed air into an air pressure chamber formed between said partition wall body and said power piston, and discharging the compressed air from said air pressure chamber to the atmosphere; wherein, when compressed air is supplied into said air pressure chamber through said relay valve part, said power piston, and therefore said output rod are moved forwards to generate hydraulic pressure in said master cylinder part, and said hydraulic pressure is applied to a wheel cylinder.
However, in the above-described pneumatic-hydraulic brake booster, the cylindrical casing is closed at the side of the partition wall body opposite to the power piston. In other words, the cylindrical casing is cup-shaped. The relay valve part is arranged at the bottom of the cylindrical casing. The assembling operation for the relay valve part requires much labor. The manufacturing efficiency is lowered. It is troublesome to disassemble or adjust the relay valve part, or to check the quality of the assembled relay valve part.