When I have problem with bleeding brakes this is what I do.

Plug both ports on master The MC is fully bled out when you can no longer stroke the MC piston inward more than about a ¼” as the fluid simply has nowhere to go and fluid will not compress.

IF you have a rock hard pedal with the MC ports plugged, then you can consider the MC OK and should next only reconnect the front brake circuit. If the pedal is still good and firm with only the front brakes connected, then you know that the MC and front brakes are not the problem. Next, reconnect the rear brake circuit and see how the brake pedal feels – if it has gone into a bad state, then you have at least isolated the problem to the rear brake circuit (which is usually the case).