The principle behind this quick modification is simple. Once you close the housing, you can test the integrity of your O-ring seals by pumping out some air and creating a vacuum inside.
If your housing is properly sealed, the vacuum will hold. If it's not properly sealed, then air will leak in, and the vacuum won't hold.
By watching the pressure gauge on the vacuum pump, you get immediate visual feedback.
I have to admit I was a bit sceptical about the practicality of pumping out air every time I open and close the housing, but after a few weeks with this little modification...I love the peace of mind I get from knowing with 100% certainty that my housing is closed properly before I head out on the water.
Contact David if you want him to install one of these in your housing.