Word of warning when you service the manual buttons. When you screw the hose nipples back in give them a gentle tweak with a pair of grips to make sure they're in tight.
Driving home after a dive one day I heard a <pop> in the boot. Nothing dramatic as the cylinders were turned off.
When I got around to stripping the unit down found that the O2 nipple had worked loose & had popped out.
It's something I check fairly often now as it would have been a real PITA to put back together underwater. Not to mention the way LP hoses dump gas at a hideous rate.