Ed, I am not an expert on why detectors cannot "hear" leaks in different materials, but surmise since sound is a vibration, a leak in the side of an air bag is dampened by the rubber material, this making the vibrations or sound not detectible by the ultrasonic detector. Trust me. It can't be heard.

About why no single way of detecting leaks is suitable....you have to go through the process. First you assume leaks are audible. Not always. Not with your ears, not with the stethoscope, and not with the ultrasonic detector. But that silent leak may blow bubbles in a soapy solution, such as on the side of an air bag.

Not all leaks are big enough to hear. Some are audible by a stethoscope, an especially good tool to get you closer to the source when the sound is echoing off various surfaces. Your ears and the stethoscope and soapy water may never detect a bad Norgren five port back by the drive axle that is exhausting the leak in the steer compartment. (That is the flow path in my Liberty and possibly in your Vantare.) But the ultrasonic may hear that internal leak. Each device has its advantages. Soapy water is simple. A stethoscope less simple, but still has its place. The ultrasonic detector has widespread use, but just isn't going to find that leak in the air bag crimp or sidewall. Bubbles work there.

I have not tried freon as a leak detector. I have freon, and a freon leak detector. When the time comes I will pressurize my system with freon and see how the detector works compared to all other methods. I think it will prove to be another tool and not the universal answer.

Hector uses a bubbler and he posted info on that. I have not used one yet. To do so properly means it is necessary to not only rig one up, but to get all the adapters so it can be attached to all the fittings that are going to be encountered. That method is a bullet proof way to find leaks, but it involves a lot of patience and the ability to start at the system source and follow it thoughout its path. It requires an understanding of the systems. I intend to try that method also.

Leak detection is a black art, and to locate all the bus system leaks involves a lot of time, the right tools and patience. It will drive you nuts as everyone of us who has worked to tighten the system so the air compressor will cycle only once a day or less.