Spring actuated brake cylinder device with fluid pressure operable release piston lockout means

A spring-actuated brake cylinder device of the type comprising brake piston means including a brake piston and a piston rod having a larger-diameter portion at one end opposite an end connected to the brake piston and an axially disposed annular groove formed within said larger-diameter portion at the juncture of said larger-diameter portion with a smaller-diameter portion of the piston rod and extending along the radius of said rod a distance less than the radius of said larger-diameter portion, a brake spring acting on one side of said brake piston means for urging movement thereof in one direction toward a brake-applying position, the opposite side of said brake piston means being subjected to service brake fluid pressure prevailing in a release chamber disposed adjacent said opposite side of the brake piston means and acting in opposition to said brake spring for urging the brake piston means toward a release position, and a locking device which comprises a locking element capable of blocking axial movement of the brake piston means, a locking piston to which said locking element is operably connected, said locking element having a projecting toe engageable in said annular groove in a locking relationship therewith, biasing means for urging said locking piston, and therefore said locking element, toward a locked position in which said annular groove is engaged by said projecting toe, said locking piston being subjected and operable responsively to said service brake pressure at a displacement value commensurate with the value at which said brake spring is effective for initiating movement of the brake piston means out of said release position, said displacement value of said service brake pressure being greater than a locking value thereof at which said biasing means is rendered effective for moving said locking piston and element to said locked position.