jacrispy, have you tried inflating the bag while keeping the black cap on and just doing nothing but observing for a few minutes to see if there's a leak?
I've played around with the rebreather twice, using the regulator to inflate the bag with screwed-in nitrogen canisters while keeping the 'Black Cap/Seal' on and the pressure relief valve set at '5'. The bag fully inflated with spare nitrogen leftover in the canisters both times. On the second test, after inflating the bag, I let the device just sit for 10 minutes and the bag was still ~90% full. I then let the rest of the canister's nitrogen into the bag. It was like ~3 seconds of actually filling up the bag, and ~12 seconds of leaking out the pressure relief valve because the bag was overfull. Then I let it sit for 20 minutes. It was ~80% full at the end of 20 minutes, just slowly leaking out the pressure relief valve. It seems like my rebreather doesn't leak except where it's supposed to: the pressure relief valve.