I've always bled brakes without the cover on the master cylinder. I just lay a rag over the master to stop the minimal spray that might come back. Usually the rag barely gets wet. It is much easier to monitor the level during bleeding with a loose rag in place than with the cover snapped down. I wonder why you have that much back pressure when the pedal is pushed and released. It seems excessive to me. Mine don't seem to do that.