Rear brake failure

[Krawler]

Hello every this is my first post.
I have a 72 c10 with front disc brakes and a 70 3/4 drum rear, my rear passenger side brake was locking up so I replaced the wheel cylinder, it was in bad shape, and went to bleed the brakes in the back. after bleeding the air outta the new wheel cylinder I found the master cylinder was empty and filled it back up, now I have no pressure to the back brakes but plenty to the front. I also have a trailer brake setup on my truck mounted under the steering wheel and it wasnt getting pressure there either. Is it my proportioning vavle, or can 1/2 the master cylinder go bad?

Thanks