here is another way to check for leaks if you do not have a sniffer. Mild dish soap, a cold engine and lightly soak each fitting, one at a time and look for visual bubbles and look for very tiny ones if no large leak is found. I have a sniffer and if there is any contaniments near the fitting, a false reading could be created. I do both, but the bubble system is a back up after my sniffer is used.
Still using the best freon ever created, R-12. Joe Machado