Had a problem a lot like you describe. When I installed my brake system the first time around, all new parts, the system worked fine. Car sat for quite a while before I got back to working on it . One day I noticed a drop of
oil on the rear axle housing. Further examination proved to be brake
oil. Tightened things up and went to re bleed the caliper. Couldnt get it no matter what. Finally tore everything out back to the master cylinder. Finally discovered that one of the plunger seals in the master cylinder was not quite doing its job.
Long story short, put in another new master cylinder, reconnected the lines, bleed the system and a week later I had brakes again.
Aaron