Sure sounds like there must be air left in the master cylinder ,I have had luck bench bleeding MB masters by finding thick walled vacuum tubing or rubber fuel line that is slightly bigger that the brake line outlets on the master cylinder and screwing the tubing in all three outlets and putting the other end of the tubing in the fill hole in the master culinder resivor which of course is full of fluid ,sometimes takes 50 or more pumps to get all the air out............

William Rogers.......