[Richard and Rhonda]

Yes, and sub freezing temps make mine leak like a sieve.

I always suspected the push on connectors not sealing well in cold temps.

I don't know about the ability to detect air leaking back through the compressor. It could be possible that the compressor internals muffle the ultrasonic whistle the detector is looking for. The check valves are notorious for leaking. If you want to be sure. Unscrew the line from the compressor, pressuize the system, and put the sniffer on the check valve.