Anti-lock vehicle brake systems

Claim 1

An anti-lock vehicle brake system of the type used with a vehicle wheel and an associated wheel brake including a fluid pressure sourse which in response to a braking action provides fluid under pressure by a fluid pressure connection to the brake, the fluid under pressure being effective to produce braking pressure at the brake to cause the letter to apply a braking force at the wheel, a solenoid anti-lock control valve having an inlet in fluid communication with the brake, wheel movement sensing means producing an output signal causing the anti-lock control valve to open when a predetermined relationship exists which signifies when wheel locking is imminent, and a relief connection in fluid communication with the brake and the outlet of the anti-lock control valve, opening of the anti-lock control valve reducing fluid pressure in said brake by allowing displacement of fluid away from the brake into the relief connection, the improvement which comprises in said relief connection at the outlet side of said anti-lock control valve a fail-safe arrangement for comparing the fluid pressure at the outlet side of the anti-lock control valve with the fluid pressure at the brake, said fail safe arrangement comprising: a. means responsive to fluid flow at the outlet of the anti-lock control valve due to the anti-lock control valve being at least partially unsealed at the onset of brake pressure application when the anti-lock control valve is unactuated whereby there is pressure build-up at the outlet of said anti-lock control valve, the fail-safe arrangement sealing the outlet of the anti-lock control valve and b. if there is no fluid pressure build-up at the outlet of said anti-lock control valve at the onset of brake pressure application, said fail-safe arrangement being responsive to the fluid pressure at the brake to unseal said relief connection and thereafter maintain it unsealed notwithstanding subsequent build-up of fluid pressure at said outlet side due to actuation of the anti-lock control valve responsive to the wheel sensor.