Cozumel, last November, my nearly new Mark 11 quit delivering air. Sure, I could get an occasional partial breath, the BC would not inflate. I made a thread on scubaboard. The cone shaped sintered filter was nearly fully occluded by a tank O-ring that was shoved inside. The tank O-ring was in place, the Mark 11 Scubapro cap does not have the "spare" O-ring extra. Only two things could have happened, the O-ring from the previous tank was in the cap and when I installed and I did not notice an extra O-ring pinched in the valve to regulator assembly, an O-ring was inside the dip tube or inside the valve of the tank. The tank still had plenty of air, it functioned with another regulator just fine.
It was a night dive so upon removing the Mark 11 I did not see anything unusual. I was tired and went to dinner and then to asleep. Next morning, thinking I would need to pull my spare reg set out, I took a close look at the Mark 11 and found two things. The O-ring shoved inside the sintered filter and the hose was loose but not leaking.
The Mark 11 has functioned normally ever since and has probably now 100 dives or more since then.
