The present invention relates to a technical field of a brake system in which a master cylinder (hereinafter, sometimes referred to as "MCY") is operated by a press of a brake pedal to develop fluid pressure (hereinafter, sometimes referred to as "MCY pressure") which actuates a brake cylinder, performing braking operation and, more particularly, to a technical field of a brake system which is designed to have a very short stroke of a brake pedal as compared to a conventional one and has suitable braking characteristics.
Among brake systems for automobiles, a brake system in which the braking action is carried out by fluid pressure converted from a pedaling force exerted on a brake pedal is well employed. FIG. 9 is a schematic view of the basic construction of such a conventional brake system using fluid pressure. In FIG. 9, numeral "a" designates a brake system, "b" designates a brake pedal for braking operation, "c" designates a tandem-type master cylinder (MCY) for generating brake fluid pressure, "d" designates a primary piston of the MCY c, "e" designates a secondary piston of the MCY c, "f" and "g" designate return springs, "h" designates a primary fluid pressure chamber of a first brake circuit, "i" designates a secondary fluid pressure chamber of a second brake circuit, "j" designates a reservoir for storing brake fluid, "k" designates a primary-side fluid line for the brake fluid between the primary fluid pressure chamber f and the reservoir j, "m" designates a secondary-side fluid line for the brake fluid between the secondary fluid pressure chamber i and the reservoir j, "n" designates a brake fluid pressure line in the first brake circuit, "o" designates a brake fluid pressure line in the second brake circuit, "p" and "q" designate wheel cylinders (hereinafter, sometimes referred to as "WCY") in the first brake circuit, and "r" and "s" designate WCYs in the second brake circuit.
As shown in FIG. 9, in this brake system a, the primary piston d of the MCY c is advanced (to the left in FIG. 9) from the illustrated inoperative position by a pedaling force exerted on the brake pedal b. As a sealing portion (not shown) of the primary piston d passes an opening k.sub.1 (for communication with the primary fluid pressure chamber h of the MCY c) of the primary-side fluid line k and further advances, MCY pressure is developed in the primary fluid pressure chamber h. By this MCY pressure, the secondary piston e advances so that a sealing portion (not shown) of the secondary piston e passes an opening m.sub.1 of the secondary fluid line m and further advances, MCY pressure is developed in the secondary fluid pressure chamber i. These MCY pressure is introduced into the WCYs p, q and r, s through the brake fluid pressure lines n and o, respectively, thereby carrying out the braking action.
In this brake system a, as mentioned above, the pedaling force exerted on the brake pedal b is converted to the fluid pressure by the MCY c and the WCYs p, q and r, s are actuated by the fluid pressures, thus carrying out the braking action.
As for brake systems, recently, it is desired to design the stroke of a brake pedal as shorter as possible in a view of the facility of the braking operation.
In the aforementioned brake system, however, a predetermined amount of brake fluid must be sent from the MCY c to the brake circuits until fluid pressures are developed after the sealing portions of the pistons d, e pass the openings k.sub.1, m.sub.1 of the respective fluid lines communicating with the reservoir j, due to the idle distances of the WCYs p, q and r, s, and so on. Accordingly, the pistons d, e must advance largely in order to send the predetermined amount of brake fluid. Since this action does not directly actually develop the fluid pressures, this action is idle travel. The stroke of each piston d, e includes the travel from the inoperative position to a position where the sealing portion passes the openings k.sub.1, m.sub.1 of the fluid lines k, m communicating with the reservoir j and the idle travel, i.e. a large stroke. As a result of this, the pedal stroke of the brake pedal d is inevitably large. Concretely, the pedal stroke of the conventional brake pedal is generally in a range from 60 mm to 80 mm, even the shortest one is about 40 mm.
The above mentioned desire for designing the stroke of a brake pedal as short as possible can not be sufficiently fulfilled by the conventional brake system.
Brake characteristics of a vehicle including (i) feeling of braking, (ii) feeling of response, and (iii) feeling of pedaling are determined by three factors: (1) pedal characteristics, (2) travel of a brake pedal, and (3) deceleration (brake fluid pressure) of the vehicle. When the shortening of the pedal stroke is enabled, it is desired to optimize the brake characteristics by taking these three factors into consideration.