Even though you rebuilt the master it still sounds like that is probably where your problem lies. If you're properly bench bleeding, (zero bubbles) and that much air is getting into the system that quickly (without even driving), it really couldn't be anything else. Try to get a professionally rebuilt one or even a new one if possible. if that dog won't hunt, I would bail on that setup. It's notorious for having problems. I had a set on my 68, pulled them off and went with the 69 single piston setup about ten years ago. Double piston's have been sitting in a box in storage ever since. Have never had any issue's with the 69's. Have wildwood disc all around on my 69 bird. It needs the extra stopping power though.