Hmmm.. I'm trying to remember, if you turn the adjustment 'in' I believe, be 1/4 turn increments, should go down about 10 p.s.i. per turn.
Don't hold me to that, you may have to come out with it, but I'm certain on the 1/4 turn though. Either way, it should be easy to tell if you're going the right way or not. If all else fails you can put the old governor back on.
150 is a little high, yeah, but not dangerous or anything, the application pressure will be dependent on the pedal position and the treadle valve.
I used to service air dryers on the trucks I drove, I never had an experience like yours though. The most common problem was in the winter the dryer would freeze and not cycle, hence it wouldn't let the truck build any air (like yours).
We usually ended up bypassing the dryers until warmer weather came around.