The kits that we supply use very expensive and ultra reliable fittings. Leaks are almost unheard of. We regularly install exotic air suspension installs with over a hundred unions (multi controller systems and simulators) and experience no leaks on the system. We could get far cheaper unions which would save a little on kits but any saving would quickly be lost in additional time spent chasing leaks. I still remember the early days in the early 2000’s when we used those cheaper unions and it regularly took 2-3 days to find all the leaks (few unions did not leak!). You can read about those experiences elsewhere on this site.
A client recently brought us a vehicle with air suspension from another installer and cheaper unions – you guessed it multi leaks. We usually use our own leak test but this time tried some expensive gas leak detector spray. It failed to find leaks that our 99p home made detector could! We use a 99p pump action plant sprayer (you could recycle a kitchen cleaning product container if cleaned well). Fill it mostly with water and about a tablespoon of washing up liquid (you don’t need much). Systematically spray every union and you will soon see bubbles coming from any leaks. Its quick, its simple, and its effective.
Its advisable to do an overnight leak test monthly and if this shows a leak however small deal with it before it gets worse!