Very simple one-way or check valve with adjustable cracking pressure. The two ports are currently 1/4 inch but I am also including the SolidWorks files in case you want to modify or remix it. With the right spring, you can very easily blow in one end but not the other. You need to print this at 0.1mm especially the threaded plunger and nut. I printed this on a Rep 2 using PLA at 0.1mm and 100% fill for the plunger and nut and 5% for everythign else. The two end caps were printed with the 1/4 inch port facing down and using a raft and supports. You will also need a small spring and can adjust the nut to provide the necessary cracking pressure.