The actual systems used already check valves, however they where not absolutely leakage free.
As an improvment the Flex Check FR080Z-000 have been implemented with a througput of 2 gramm liquid CO2 per second.
The new modul has been tested in Europe and then send to the US for further test by the NASA. In April 2019 the modul has been succesfully sent and implementetd on the ISS.
