The nut came loose and fell off inside the counterlungs. My Revo class was taught by Paul himself. This possibility certainly wasn't covered in class, nor is it part of the build checklist.
As to your leaky valve question, I don't know any leaky valve without the limitation, which is why I'll never own a leaky valve ccr.
Usually 10/65 or 10/70 at 300'ish feet. It's not the helium, it's that back mounted counterlungs suck. Anyone who has spent serious time on bmcl and then switches to fmcl never tries to this point. Again, dive out of trim, and it's not an issue.
Thanks for the response, I still don't understand the WOB issue, in trim or pretty much 25 degrees either way it breathes fine to me, its when you are vertical on the surface waiting to be picked up or on the diver lift when the WOB is crap.
Regarding the OPV I had mechanical damage to my OPV after a flight and when I tried to do a + or - test the unit wouldn't pass and that's how I discovered the issue. A nut coming loose is certainly a strange one, cant say I've heard of it happening before or since and especially with it passing the pos/neg tests while being loose.