Before Christmas Mark from Heidelberg West in Victoria had a Holden VE Commodore drive into his workshop. There was no visual evidence of dye or oil or any sign of a leak, so the system was re-gassed and sent on its way.

Then after Christmas, the vehicle was back in his workshop with the a/c not working and this time he could see a dye spray from the manifold hoses on the compressor - a giveaway the compressor seals were leaking. Having replaced the seals, he pressure-tested the system with nitrogen again - however after an hour and a half it was clear that surprisingly the system still wouldn't hold pressure.  

In desperation he wound up the Nitrogen pressure up to 350PSI but still no bubbles appeared anywhere, and still no trace of UV dye!

“After wasting time and mucking around” he pressurised the system with Hi-Trace to 75 PSI

and pulled out his Loktrace leak detector. Within 10 minutes, the Loktracer tool had “sniffed out a hair line crack down the metal pipe on the discharge hose, about 50mm from the manifold.” Shortly after the pipe was fixed and the customer was good to go.

There was no chance he could've found it otherwise, “100% no dye and even at 350 PSI couldn't find it with nitrogen despite being in plain sight” and his only regret was that he “Could've saved so much time and effort if it [the Loktracer] had been used to start with!”