My '61 has a Moraine power brake booster that's tits-up.
Wondering if there's a modern economical direct replacement that bolts up?
Chevelle?
It's still using the original style single pot master cylinder.
Have Booster Dewey ( his place is in Oregon) rebuild your unit. I have 3 to 4 rebuilt originals by Booster Dewey on my 62's they work great. He may have a core so you could trade. I think I have both types used.
Paul
If you live in or near a large city just find a rebuilder to do it. While Booster Dewey does good work you can have it rebuilt much cheaper at regular rebuilder shop