John Guest Male Connector NPTF - 3/8 x 1/8 NPTF

they leak

The john guest fittings rely on an o-ring that is compressed when the darker ring (and the tube) is pulled out. Unfortunately, it is prone not to stay pulled out, letting compression off the ring, and then it leaks. I have seen some with clips that get inserted to prevent it going back, and assume this would be the answer. Look for ones with the clips to keep the thing out. I don't know if the clips can be bought seperately.