Something I have written in past 30 minutes have all gone. Internet is very slow here via nuro.
When the beans being heated up they release certain amount of steam and when they reach to the boiling point the pressure in the drum drastically raise up, because the volume of steam is more than 1700 times the original liquid. It is more than enough to bring up the pressure inside the drum. It means that it cannot be less than atmospheric pressure when the ventilation is properly done. Of course, it is true that the positive pressure in the drum could sometimes mean lack of exhaust air flow and leads to some defects on the roasting but when the pressure becomes apparently negative, it means that you are loosing calories because of the excess air flow that it is not what we desire since the fire of the Fuji R103 is not so strong and you may be loosing aroma coming out of the beans especially in the mid of the roasting stage.
Plus, if you are loosing too much vaporated water in the drum at super heated steam stage, the vaporation within the beans may slow down prolonging the roast time. So, I think at least when you work on the roaster like Fuji you need to watch over the pressure in the drum to keep it at some optimum range after the beans reach boiling point until just before the second crack begins. It is best to employ some automated system to keep the pressure in the drum just like giessen and yamato do.