I would like to paint using a black brush with a 40 percent oppacity, in a way that even when I finish the brush stroke and then repeat it in an already painted area, the 40 percent oppacity level will not become darker..
Sure, paint on a new layer that contains only the black. Use a brush with opacity set at 100%. Cut the layer opacity to 40%. When you are SURE that you are done painting, merge the layer with the layer you want it to be applied to. Or, never merge the layer as long as you save in a format that preserves layers.