[Eman]

You put the MC in a loop and plugged the ports and the pedal is firm so there is no air in the MC. You plug one port and hook up one end of the brake system and the pedal is good. But when you hook up both ends the pedal is low. Is that correct?
If all that is correct, I would try plugging the lines at all 4 calipers and see how the pedal feels. If it's good then I would say the MC is a little small and not moving enough fluid to make up for the amount displaced when the pistons retract. Larger MC or residual valves should help.