This is a check valve designed for a 3/8" hose. I use two of them in combination with a squeeze bulb to move water (from the boat bilge to a bucket).

I printed the parts using ABS and fine settings on CURA. The small, conical shaped plunger is inserted in the part with exterior threads. The point of the cone goes toward the small opening. The channels on the edge of the cone ride on ridges of the exterior-threaded piece. I use an acetone dampened cloth to wipe the end of the cone and the mating surface inside of the small piece with the exterior threads to polish them. These two surfaces provide the seal to ensure one-way flow.
With the cone inserted on the exterior-theaded piece, screw on the outer piece (with the arrow) over the piece with the exterior threads. Be sure to keep the arrow pointed up and the cone pointed down as you screw the two pieces together so that the plunger stays in the bottom fo the smaller piece.
Water will only flow one way, in the direction of the arrow (away from the point of the inner cone).