If you have a piston problem there can be 2 things: a) they leak or b) they jam (improper retraction ususal because of dirt or corrosion). It seems improbable to me that a sagging pedal is related to these pistons if you are not experiencing one of the above. If you still have the problem and there does not seem to be air in the system anymore (bubbles coming out), I suggest you go back to the basis of the troubleshooting.
To my opinion you would have to pump the m/c empty, remove the reservoir, check the position and stroke of the primary piston and check if the stop pin of the secondary piston is present. If necessary remove the m/c and check if the input motion of the brake pedal is correctly transferred through the brake servo. I.e. let someone stop the output pin and see if you can move the brake pedal down. If all is correct there should be no play between these 2 parts.