Hi, sorry, a bit late to the party, but for anyone else trying to hunt down difficult leaks on their air system, you may want to look at air escaping back through the non-retun valve and out through the pump. I've spent the best part of two weeks trying to locate a leak on my new twin gauge air suspension assist install on my 2012 Ducato. It's a good kit with everything you need to complete the task, so it was a straight forward job, with the only delay was releasing the old bumpstops.
Overnight, the nearside system lost nearly a bar in pressure. After much work, systematically clamping off the airline at different points I thought I'd finally found that the gauge was leaking at the union, so I remade the joint with locktight jointing compound and after the prescribed time, repressurised the system and it seemed to be holding, but the pressure was down by just over a bar the next day.
So a call to the supplier was made, and after laying out everything I'd checked and rechecked, he suggested adjusting the non-return valve, something I hadn't considered as it looked like a sealed unit. But with every other avenue explored and much leak test fluid spent to no avail, I disconnected the compressor line from the system, and after a quick squirt of test fluid into the open end of the line to the non-return valve, this revealed that the valve wasn't holding and air was leaking backwards through the compressor and out to air. And because this was all happening inside the pipework, there was no obvious indication of this that any amount of test fluid would have revealed the leak.
Following his advice I opened up the valve, and sure enough, it's possible to tighten down on an internal spring loaded valve and O ring seal, to increase the pressure against the inlet to seal it properly. The internal valve was quite loose and was letting air past the seal. I used a pair of fine needle nosed pliers to reach in and tighten the brass insert down to apply more pressure onto the spring, which in turn exerted more pressure on the O ring seal.
This cured the leak and everything appears to be holding pressure now.
In conclusion, I wish I'd investigated the non-return valve in the beginning as it would have saved much messing about, but with 24 separate joints to make, there's much potential for leaking joints and possibly faulty parts to check and recheck, so it's easy to overlook something that appears to be a sealed unit when you begin to check your own workmanship.
The other point I would make, is that although the supplier was really helpful and ultimately provided the answer to my issue, a short addition to the brief instructions included in the kit detailing the need to check and adjust the pressure in the non-return valve if a leak develops would be extremely useful.
I hope this helps someone in the future as it's a simple thing to overlook, and a nightmare if you can't track down this hidden fix.
