Am not sure where you base your observations diverdoug
One thing is progressive flooding by failure of an o'ring or a small part that is not sealing (rare and would be detected by the leak detector before you even enter the water)
The other is when the housing gets knocked by shocks and bangs, those have little to do with moving an oring but could crack a port or the lcd viewer and in that case because of negative pressure your housing floods faster than it would normally
If your housing is water tight that 200mbar negative vacuum applied makes no difference whatsoever and does not make the housing more watertight than it already is however if you had to crack a port and your housing is damaged to get less watertight the vacuum floods it faster
Eventually if you crack a port your camera is fried in either case vacuum or not
Diverdoug (and the others) are quite right
An o-ring will only seal correctly when under pressure, this pressure can be created either mechanically (e.g by clamping down the latches on the back of a housing and by the surfaces of the housing being forced together as the housing is submerged) and/or through fluid/air pressure (e.g through sucking air out of the housing, or by submerging the housing in water)
A pressure based leak detector assists with this by both pulling the surfaces of the housing together creating a higher mechanical pressure, and by applying air pressure by creating a pressure differential between the inside and outside of the housing.
If a camera and housing is left bobbing on the surface untouched then unless it's very poorly made or has a fault then I doubt very much it would flood. However that's not what happens in the real world, housings naturally get banged by waves, against the divers person etc. As the o-ring has minimal activation through pressure at that point, all of those physical knocks can break the seal and cause a flood. You only have to look at the amount of cameras that get flooded when left in the dunk tank after a dive to get an idea.
On the flip side, add a pressure based leak detector to your housing, you wont be able to open the housing at all once the negative has been pulled.
Edited by bottlefish, 24 March 2014 - 04:26 AM.