I’ve got an 04 Honda Civic EX - Automatic. 128,000 Miles 

I’m getting an intermittent brake pedal pressure. Sometimes it has solid pressure and stops great, other times the pedal is straight to the floor. I replaced the master cylinder a year ago and it worked great until now. I just did new rear wheel cylinders as the bores were seeping fluid. No other external leaks. I’ve pressure bled the system using both closest - furthest from master cylinder and furthest to closest methods, about half a gallon of fluid bled through in total using a Mityvac that pressurizes straight from the master cylinder cap. Still the same symptom. Do I suspect my new master cylinder has failed internally? I’m at a loss. 

Thank you!