This thing is a one-directional valve that prevents backward flow. The functional part is a piece of rubber which shuts off a small hole depending on the airflow direction.
Many sockets are available. Besides M5 and 4 mm (with or without locking ring) sockets, a bike pump socket can be installed. This thing is ideal for inflating PET bottles with a bike pump.
Oct 30, 2018: Added version 2 of bike pump socket, with slightly longer design and grooves around the nozzle to prevent getting loose. Also added version 2 of valve body with thinner cross lines to make seal more reliable.
Print valve body and two sockets that fit your application. Cut a disc with diameter approximately 10 mm from a piece of 1 mm thick silicone rubber or equivalent. The exact shape is not critical, as long as it is able to move up and down in the housing and the center is well covered. For reliable operation in multi-valve assemblies it is recommended to get a nice round shape though. Grind the rough top face of the housing until it is smooth.
Apply blue silicone (or equivalent) to the top and bottom faces of the housing. Place rubber seal in the gap. You do not need as much sealant as pictured, it is better to keep the silicone seal free of sealant to avoid sticking to the body.
Assemble with six M2 x 12 mm bolts and nuts and prepare the socket (M5 in this case).
The valve is now ready!
A 4 mm socket with locking ring is also available. No external components or glue is needed besides the 4 mm tube and 3D printed parts.