Anti-skid braking system for vehicles

Claim 1

An hydraulic braking system for a vehicle comprising a plurality of wheel brakes, a pedal-operated master cylinder controlling hydraulic fluid under pressure to said wheel brakes, skid sensing means for providing a control signal when the deceleration of a braked wheel exceeds a predetermined value, a plurality of modulators each defining a variable volume modulator chamber, a common fluid pressure source for actuating all of said modulators, a respective inlet to each of said modulators chambers connected to said master cylinder, a respective inlet valve controlling each of said inlets and maintained normally open by fluid pressure from said common source acting across a respective movable wall, a respective outlet from each of said chambers connected to a respective one of said wheel brakes, means responsive to said signal for operating said modulators each modulator when operated closing a respective one of said inlet valves and increasing the volume of its respective modulator chamber, a respective connection between said master cylinder and a respective one of said wheel brakes by-passing the modulator associated with that wheel brake, a respective normally closed by-pass valve controlling each of said by-pass connections, a respective normally open isolating valve provided on the downstream side of each modulator output, each of said isolating valves being combined with a respective one of said by-pass valves to define a single double-seat valve, and all of said by-pass and isolating valves being combined in a single valve assembly comprising a housing, a safety piston working in a bore in the housing and subjected to fluid pressure from said common fluid pressure source, a respective control piston for each of said double seat valves, said control pistons being arranged side by side in bores in the housing parallel to the safety piston bore and being acted upon by the safety piston, each of said double-seat valves comprising a first valve seat located between a respective inlet connected to said master cylinder and a respective outlet connected to a respective brake, a second valve seat located between a respective second inlet connected to the respective outlet of the respective modulator and said respective valve outlet, a respective valve member for engagement alternatively with said first and second valve seats, all of said valve members normally being held in engagement with said first seats and spaced from said second seats by said safety piston to hold closed said first by-pass valves and to hold open said isolating valves, and resilient means acting on said safety piston in opposition to the fluid pressure acting on said piston whereby in the event of a failure of said common fluid pressure source the force holding said valve members against said first seats is relieved.