with co2 tube not plug in of course it will flow out...
if you worry, can plug into air pump and see if it ok...
but to ensure you that the reactor is working fine...
just need to have a 1way check valve.
looks like this
http://www.aquacave.com/co2-check-va...-962-3291.html
cost $1.50 to $3 each I think..