Vehicle anti-lock brake system

An anti-lock braking system for a wheeled vehicle having at least one wheel provided with a wheel brake, a brake pedal operable by the vehicle driver, and a master cylinder actuated by the brake pedal and having an outlet connected to supply pressurized brake fluid to actuate the wheel brake, said system comprising:

• a fluid pump having an inlet and an outlet;
• a two position, three-way valve having a first port connected to said wheel brake, a second port connected to both said outlet of said pump and the outlet of the master cylinder, and a third port connected to said inlet of said pump;
• said valve only moveable between a first position wherein said first port is connected to said second port to increase pressure to the wheel brake and a second position wherein said first port is connected to said third port to decrease pressure to said wheel brake;
• control means for operating said valve to control the application of pressure to said wheel brake, said control means including means for cyclically moving said valve between said first and second positions during a predetermined time period to alternately increase and decrease pressure to said wheel brake, said control means operable to modulate the time periods during which said valve is maintained in said first and second positions to control the effective pressure to said wheel brake to obtain a net increase, decrease, or constant pressure to said wheel brake during said predetermined time period to prevent lock-up of the associated wheel.