Hi Darren,
As Fiona says make the first dive empty, just to be sure there are no leaks. There is no need to grease every dive, I usually do it once a day before the first dive - I have only used my housing on Liveaboards so far.
Just use the absolute minimum to coat the O-ring, too much will attract dust, dirt and hairs (fatal!). Also, be very careful not to stretch the O-ring too much, just put a little grease on your thumb and first 2 fingers, make a Triangle and gently pull the ring through. Go around a few times and carefully check that the O-ring has a shiny finish all over. CHECK FOR ANY HAIRS OR DIRT!
IT IS NOT THE GREASE THAT MAKES THE SEAL! The grease just acts as a lubricant.
When just under the surface look for bubbles escaping (means water is getting in), keep the port facing down as you descend and periodically look upwards into the port for signs of water. If there is any, get back to the surface as quickly and as safely as you can.
Happy shooting.
Steve.
