Anti-lock vehicle brake systems

Claim 1

In an anti-lock vehicle brake system of the character referred to which includes a master cylinder, a wheel cylinder in fluid communication therewith, a relief connection in fluid communication with said wheel cylinder and an anti-lock control valve in fluid communication with said relieF connection, said anti-lock control valve selectively modulating pressure in said wheel cylinder the fail-safe improvement comprising, means for inhibiting fluid flow from said relief connection to said antilock control valve, said means for inhibiting including mechanical biassing means providing a force for biassing said means for inhibiting to an inhibiting position, said means for inhibiting being responsive to fluid pressures in said relief connection such that with said anti-lock control valve closed, fluid pressure in said relief connection due to a braking action and above a certain minimum value determined by the force of the biassing means causes displacement of said fluid inhibiting means to an unhibiting position at which it cannot impede flow of fluid through said anti-lock control valve when the latter is actuated while said fluid pressure exists, and with said anti-lock control valve actuated in the absence of said fluid pressure, a fluid pressure drop across said fluid inhibiting means arising from a subsequent braking action causes said fluid inhibiting means to remain in said inhibiting position in which it severely restricts flow of fluid through the actuated anti-lock control valve.