1. Field of the Invention
This invention relates to an oil pressure booster, and more particularly to an improvement of an oil pressure booster which is used in a fork lift, a truck, an automobile, or the like vehicle.
2. Description of the Prior Art
An oil pressure booster of the prior art, which is used for instance in a fork lift, comprises a throttle valve formed of a spool to be driven by a brake pedal, which throttle valve throttles the passage of operating oil flow being delivered upon depression of the brake pedal for generating an oil pressure upstream thereof, a control valve formed of the aforesaid spool, a return path valve formed of said spool and normally communicating a boost chamber to a drain port therethrough, and a power piston operatively related to said spool. The aforesaid oil pressure is applied to the boost chamber through said control valve for moving the power piston in the direction of depressing the brake pedal so as to follow the movement of the spool. The oil pressure booster of the prior art has a shortcoming in that its operating characteristics with an unoperated power steering connected thereto (open characteristics) is widely different from its operating characteristics at the time when the power steering connected thereto is in operation (closed characteristics). Thus, when the power steering is not operated while the oil pressure booster is operated, if the power steering is put into operation for shifting the open characteristics to the closed characteristics, with a certain depressing force applied to the brake pedal (which relates to a boost chamber pressure), the oil pressure delivered to the boost chamber under the closed characteristics is considerably higher than that under the open characteristics, whereby the pressure difference under the two characteristics tends to push up the brake pedal (to be referred to as "oil pressure interference"), which push up cause a discomfort to the driver.