What was amount of the vacuum you were using for your camera? I always wondered if too much vacuum was not good for the seal. On the other hand would it be better to have the vacuum at a level that was just above the point where the green light will change to red so that a very small leak would be detected as the vacuum decayed slowly away? The dial on these vacuum gages are broken up by one half inch increments of vacuum and therefore the resolution is not all that good for detecting very small changes in vacuum readings. I don't know if a vacuum gage with more precision would be a better way to help detect a very small leak.
The other feedback I have on this topic: is anyone recording the vacuum when first setting up the system and recording the vacuum just before a dive to see if there is a slow leak? I usually draw a vacuum overnight and check it again in the morning but from Udo's experience I am not sure this would catch such a small leak. Has anyone caught such a small leak before by performing a vacuum drop test (i.e. check/record the vacuum over a period of time)?
By the way thanks Udo for the feedback on your experience. I think most people have gotten the impression that these systems will detect leaks in all circumstances. This obviously is not the case in your circumstance. I will certainly be more sensitive to the indications of moisture buildup inside the housing now that you have you have shared your experience.
I use an altimeter inside my housing, with the camera installed, to monitor vacuum. If you have insufficient space for that, I think placing one inside an empty housing could be useful in diagnosing small leaks. My housing has never leaked so I can't say for sure, but if a control button leaks a small amount of water, then wouldn't you expect that it would leak a larger volume of air; And if you manipulated that button isn't it likely that the altimeter would display a detectable loss of vacuum?
When I was looking for my altimeter, I considered a more expensive didital one that claims to register 2 ft inrements in altitude (the reading changes If you raise it from your feet to eye level). This altimeter is called the MicroTim, and it costs about $200.
A small aneroid barometer/altimeter costs much less. Here is a cheap one from China that was too big for my purpose but was otherwise surprisingly well put together - I know because I destroyed it while trying to make it fit. It cost less than $20.00 but took over 3 weeks to arrive: