I've been experimenting for some time on how to create 3D printable check valves. This check valve utilizes a silicone disc and ball. This silicone piece is inserted mid-print, minimizing the chances of air escaping in other spots. Once fully printed, the silicone part is put in a sprung position, allowing it to be pushed by air in one direction but not the other.Instructables guide: http://www.instructables.com/id/3D-Printed-Check-Valve/Video: https://youtu.be/PwzdimEBX2A