Valve devices for use in liquid pressure braking systems of vehicles

摘要

A valve device for use in liquid pressure vehicle braking systems to control rear brake pressure comprises a plunger in a cavity in a housing, the cavity being closed at one end. The plunger has a land which slides within a portion of the cavity. The area of the plunger that is exposed to liquid pressure at the closed end of the cavity is greater than the area of the plunger that is exposed to liquid pressure on the other side of the land. A passage through the land is inclined to the axis of the plunger. A valve unseating pin in the inclined passage co-operates with a frusto-conical surface of the cavity to unseat a ball in the inclined passage. A biassing spring surges the plunger towards the closed end of the cavity against the action upon the plunger of rear brake pressure which acts upon the larger presented area portion of the plunger. Liquid pressure is transmitted from the master cylinder to the rear brake operating motor cylinders when the ball is unseated and not when the ball is seated. The valve functions so that rear brake pressure increases at the same rate as does master cylinder pressure from the commencement of braking until a predetermined liquid pressure has been built up whereafter it is reduced in relation to master cylinder pressure. The valve may be arranged so that the pressure that must be built up before there is any reduction in the rear brake pressure compared with master cylinder pressure is higher in the event of failure of the front brake operating sub-system than it is when both the front brake and rear brake operating sub-systems are operating normally.