With road brakes, overfilling have it's advantage to provide a sharper bite, however on some MTB brakes, it's a little too sharp, sometime I have to open the port and squeeze a bit out and close it (while squeezing), this open up the piston a bit more.
I didn't know that could be a problem.
I was careful to fill it to the brim and roll the diaphragm into place, following these instructions from Epic Bleed Solutions.