Ryobi 14" chop saw c356 type III, large sparks at one brush ? New brushs. Picked this up used, was missing 1 brush so I replaced.. armature or brush problem ?

I assume the saw runs. Make sure that the brush moves freely in its holder such that it make good contact with the commutator. I think all wire wound armatures spark between the brushes and the commutator.
Your post implies that one brush is sparking excessively as compared to the other.
You could test to see if this is a characteristic of the saw by reversing the wires to the brushes. If the sparking moves to the other brush then it is a characteristic of the saw. One brush is connected to the hot side of the electric circuit and the other to the neutral. There is a voltage drop as the electrical current passes through the armature windings. That may account for the differences in sparking.

If you can't reverse the wiring at the brushes, you could temporarily reverse the wiring at the saw switch. At the switch you will find the incoming white wire or neutral, connected to the white of the saw. The black will be connected to the switch. Temporarily reverse these.
Another way to reverse the wiring, and probably the easiest, would be to find an old extension cord. Cut off the ground prong so that you can plug it in backwards with the neutral in the hot and the hot in the neutral. Then plug you saw in to the extension cord.
Remember this is temporary, for testing only. The hot and neutral in the saw is wired a certain way for safety. The saw motor doesn't really care.