I've done a similar thing with my SCBA tank for filling guns. I have a M-rod and it does fill pretty quick off the tank but I can control it. OTOH I have a Diana bandit with only a 50cc reservoir. I always hand pump that BC I'm concerned it will fill too fast off the tank and overshoot max pressure.I hand pumped my Prod at first after getting my YH. I was worried it would fill it too fast and I would over fill it. It fills it fast but not that fast. I think it took at least a minute, might have been 2. It fills my 45 minute SCBA tank (I think that's 66 cubic feet) from ~3000 to ~4000 in about 20 minutes.