Pressure-relief valve with stepped or double piston

Pressure-relief valve for the protection of hydraulic units, particularly of hydraulic face support systems in underground mining and tunnel construction against sudden overload due to rock pressure or similar causes, with a valve piston defined by an outer surface arranged to slide in a valve housing against the force of a valve spring and with an inlet opening at a first end of the valve piston provided with a blind bore therethrough and with radial bores that pierce the piston outer surface and with cross bores provided on an inner wall of the valve housing communicable with the radial bores, whereby the valve piston is sealed by an annular ring overridden by the radial bores, wherein the valve piston is formed as a continuous tubular shell with an attachment at a second and remote from the first end serving as control piston having a reduced outer diameter relative to an outer diameter of the tubular shell and with sealing rings lodged in the inner wall of the valve housing both above and below openings defining the cross bores, whereby two separate chambers are defined inside and above the valve piston both subjected to the same pressure and the control piston rests against a spring cap loaded by the force of the valve spring.