Hello
I am in the commercial refrigeration trade for 25 years and although have only worked on maybe three automotive systems, one way we look for a hard to find leak in a small system is to introduce a small amount of refrigerent into the system, then pressurize it with nitrogen (in this case about 150 psi for the low and high side), this will sometimes make it easier to find the leak with the electronic detector, then pin point it with soap bubbles.
One other trick is if you have a little bit of R22, use this for the trace gas as it is easier to pick up on the electronic leak detector than R134A.
