Could you pressurize the system, maybe to a high enough pressure so that the leaks would become more audible?
Any stop-leak you put in the system will be on the low pressure side, and would probably lift off the section surface as the vacuum intensifies.--NBC
But we really don't have any substantial compressed air capability around the building; I do wonder if a modest compressor would be able to keep pace with the leaks, though. I suppose it's worth a try. I was wondering if the stop-leak would be throwing money away.