Non-return valve

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 loos