I had a Olds 88 that would not bleed out even with a pressure bleeder. Left the master cylinder cap off and the bleeds open..... That was the only thing to ever given me such a problem..... Up, Down for years then a professional bleeder that held over 1 gallon of fluid... All disc break cups rebuilt, and wheel cylinders rebuilt. All rebuilt by me.... I have done 100's of them.
So this might be worth a try.