Valves that control and regulate hydraulic fluid pressure to constant values in hydraulic systems. Includes relief valves, pressure reducing valves, unload valves, sequence valves, and other components responsible for pressure control in hydraulic circuits of press machines, injection molding machines, machine tools, and construction machinery. Critical components that ensure hydraulic system safety and efficiency through pressure setting adjustment functions, overpressure protection functions, and pressure reduction functions. Functionally distinct from flow control valves and directional control valves.