i know cellblade that you stroke the pentool path with a brush.
but what i am keen in knowing is, how come i can see the background through the lines?
if i decrease the opacity of the line with the brush strokes the glow effect you have on it becomes light too but in this case of yours, the lines are somewhat transparent and the glow effect is good.
can you please tell me how did you get it? 
EDIT: I think i did not read your message properly. it was written "its the fill opacity not the master opacity."..got it, thanks!! 