I have a new set of Shimano Zee M640 brakes. With the bleed block in place, I squeeze the lever until I start feeling pressure (when the pistons start squeezing the block); from this point, I can pull the lever another ~10-15mm if I squeeze harder. Is this flex or is this air? Should the lever be dead stiff? Should there be a bit of flex? How much flex? Anyone of you have experience with M810/M640/M820 and lever stiffness? How about other Shimano brakes with Servo Wave? My lever felt exactly the same after a bleed, both front and rear.