I have one of those power bleeders and it works great. If I remember correctly, the book recommended 15PSI. I expect it would work with about any pressure at all, but I pump it to 15. It is most useful for flushing, I have not used one for bleeding dry brakes.
I am not surprised that your trans problem was clutch hydraulics. I went through what you've been through. It was many years ago and I just finally went to the regular bleeding method, works fine. It will work better the next day, because bubbles find their way out over night after the bleed.