Pressure control apparatus in a vehicle braking system

A pressure control apparatus in a vehicle braking system having a wheel cylinder operatively mounted on a road wheel, a reservoir for storing an amount of brake fluid, a master cylinder for receiving the brake fluid from said reservoir and generating a master cylinder pressure in response to depression of a brake pedal to supply the master cylinder pressure to said wheel cylinder, and a power pressure source for receiving the brake fluid from said reservoir and discharging a power pressure, comprising:

• a housing for defining therein a bore and having a first port connected to said reservoir and a second port connected to said power pressure source;
• a first sliding member slidably disposed in said bore of said housing for dividing said bore into an input chamber which communicates with said master cylinder to receive the master cylinder pressure and a pressure chamber which communicates with said wheel cylinder and supplies a hydraulic pressure thereto, said first sliding member sliding in response to a difference between the pressure in said input chamber and the pressure in said pressure chamber, said first sliding member having a bore defined axially therein and a closed end, and said first sliding member having a first drain port which communicates with said first port of said housing and a first power port which communicates with said second port of said housing;
• a second sliding member slidably received in said bore of said first sliding member, said second sliding member having a second drain port which selectively communicates with said first drain port in accordance with the relative position of said second sliding member to said first sliding member, and a second power port which selectively communicates with said first power port in accordance with the relative position of said second sliding member to said first sliding member, said second sliding member having a through hole defined along a longitudinal axis thereof to communicate said pressure chamber with a space defined between said first sliding member and said second sliding member, said through hole communicating with said second drain port and said second power port;
• and
• solenoid means having a solenoid and a plunger engaged with said second sliding member and actuated by a current fed to said solenoid, said solenoid means providing a set pressure in response to the current and actuating said plunger to move said second sliding member toward or away from the closed end of said first sliding member to regulate the pressure in said pressure chamber into the set pressure.