Abstract: A one-way valve for liquid sampler devices and the like which has an effective self-cleaning function for use in sampling liquid or gas fluids containing solids and sticky material. The valve and seat are in a housing where a desired liquid or gas to be sampled is passed with the valve permitting flow in one direction only. The seal will close the input to the housing when flow is attempted in the other direction and prevents same. The valve is of concave shape and made of soft pliable material such as rubber. The valve seat is essentially a flat round disc with a smooth surface and an inlet hole in the center. The soft pliable material valve lays on the seat, unattached, with the outer edges touching the flat seating surface. The concave shape of the valve allows the incoming material to exert its pressure over a large area of the valve, to give maximum lifting force to the valve. This extra force is important when sticky or gummy material is present and on the seat.