A sheet bending brake comprising a frame having a fixed jaw and a movable jaw and an anvil member adjustable secured to the movable jaw. The fixed jaw has a clamping surface movable between workpiece clamping and non-clamping positions relative to the clamping surface of the fixed jaw. A bending member is hingedly connected to the fixed jaw. The movable jaw is releasably locked in workpiece clamping position by a structure that includes an oval shaped spring device member having opposed ends and opposed walls. The spring device is pivoted at one end to said movable jaw and at the other end to a handle for manipulating the jaw and comprises a pair of links each of which comprises front and rear walls. The front walls are pivoted to one another and a wedge is positioned between the rear walls so that as the thickness of workpiece varies the spring device will pivot to accommodate the different thickness.