New Brake Bleeding
You need to get a mid 70’s “A” Body manual disk/drum brake, “NEW” not rebuilt, master cylinder. Also you need to down-size that one ton monster wheel cylinder and break assembly to an “A” Body size brake drum & 7/8” dia. cylinder. Too large a rear wheel cylinder and too much drum will cause early rear wheel lock up under moderate to heavy braking in an “A” Body.
Think about it, your car fully loaded would be about 4500 lbs., a ton truck would be well over 6000 lbs., and possibly have duel wheels with a lot of rotating mass. Over kill for an “A” Body.
You need to adapt 10 inch brakes to your axel