A check-valve for high pressure hydraulic fluids has a piston (2) sliding vertically in a cylinder (14) between stops. The valve is opened by hydraulic pressure working against a spring (3) and the weight of the piston. Fluid flows through openings (15,16) in the cylinder into an annulus (17) formed between the cylinder wall and the housing. The top of the piston forms with the housing the upper part of the annulus which gradually narrows toward the exit.The flow of fluid thus creates a suction on the piston, proportional to flow, which holds the piston against a stop formed by the upper limit of the openings (15,16) in the cylinder wall
