1. Field of the Invention
The present invention relates in general to a brake pressure control valve for controlling the brake pressure applied to the rear wheels of a motor vehicle, and more particularly to a proportioning valve adapted for use with a double piping type hydraulic braking system.
2. Description of the Prior Art
Various kinds of brake pressure control valves (or proportioning valves) are known which control the brake pressure delivered to the rear wheels of the vehicle so that once the master cylinder pressure reaches a certain value (critical pressure), the brake pressure for the rear wheels thereafter increases at a lesser rate than the input pressure from the master cylinder. With this function, the dangerous skid phenomenon wherein the rear wheels are locked prior to the front wheels under braking of the vehicle is suppressed.
Nowadays, in order to assure safety of the vehicle, a so-called double piping type hydraulic braking system is widely employed, which includes generally two groups of braking systems so that even when one system fails to operate, the braking of the vehicle is assured by the other braking system. X-piping type and J-J piping type are widely known as the double piping braking system.
Thus, when the double piping braking system requires the above-mentioned anti-skid phenomenon, two brake pressure control valves are necessary which are respectively disposed in the two (double) hydraulic lines of the system. Usually, the two control valves (or proportioning valves) are assembled in a single housing from the standpoint of mounting space and vehicle assembly, and some are of a type of so-called back-to-back proportioning valve assembly (which will be referred to as back-to-back proportioning valve, hereinafter) in which two proportioning valves are aligned in the single housing in a mirror image relationship. (One example of this type valve is disclosed in Japanese Patent Application First Publication No. Sho 55-76733). Furthermore, in order to assure safety of the vehicle upon brake trouble, some of the valves have therein a fail-safe system of a free piston type which functins so that when one braking system fails to operate thereby creating a pressure difference between the two braking systems, the free piston means is shifted, due to the pressure difference, from its pressure-balanced neutral (or home) position to a fail-safe position causing the other braking system to brake the vehicle bypassing the associated proportioning valve.
However, some of the back-to-back proportioning valves having the above-mentioned type fail-safe system fail to have a desirable self-centering characteristics of the free piston means. That is, when the brake pedal is rapidly released, there is produced, but for a moment, a prssure difference between the input pressures of the paired proportioning valves causing displacement of the free piston means from its home (neutral) position. However, due to their inherent constructions having unavoidable dimensional errors in production, some of them can not carry out rapid self-centering movement of the free piston means even when the pressure difference disappears thereafter. The rapid self-centering of the free piston means is quite important because if the brake pedal is depressed with the free piston means being not centered, the brake pressures applied to the rear wheels become unbalanced thereby causing unstable braking of the vehicle.