From my experience having got the "leaking" issue over the years in diferent customers, there is one detail that no one commented, and it is that the 4mm tube and 5/32 tube are apparently similar, same 1/4" tube and 6mm tube. I have seen festos "leaking" just because the customer mixed imperial and metric sizes. This, plus the badly cut ends, plus some brands of fast fittings or tubing that degrade quickly when used in Kero, have created the "urban mith". Festo brand with quality Festo tube, installed correctly is OK, other brands not.
Gaspar