Am I reading this right? Did you close off the splitter valve so that no gas could escape the fermentation bottle? If you did then you were VERY lucky that you didn't end up with a yeast surprise. The suprise would have been just how far an exploding bottle of yeast mixture can throw its contents! Even if you are sure that the yeast is dead you should never shut off the escape route for the CO2. The pressures produced by yeast can be increadibly dangerous. If you think the yeast is dead dump the contents or leave the top of the bottle open.
Moving on, if you came back to the yeast and there was a pressure build up like you say then the yeast was not dead. Yeast is a living organism that is stored in a near suspended animation state. It needs time to get going (just like me in the morning). You may have inadvertantly killed off a lot of the yeast with too high a water temp or something. If any yeast survives the initial cause of the die off then it will quickly start reproducing and soon you'll have a nice healthy colony again. Obviously the more yeast the faster the CO2 is produced. This is probably why there appeared to be no CO2 production to start with but then when you returned to the bottle it was under pressure.
When I used to use CO2 I would initially leave a freshly made up bottle with the top off for 24 hours to allow the yeast to get going. It would froth loads and I often got froth coming out of the top of the bottle. Once it subsided I'd attach it to the system and leave it to get on with its job. I never had a problem with failed yeast cultures.
WK
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