I just replaced the wheel cylinders on my early '71, hoping to finally fix the brake problems. Long story short, it didn't work.
I think the problem being the wheel cylinders don't slide in their slot. Just learned that's how they're supposed to work. Everything is new and clean. I've even wire brushed the plates and coated everything in a generous layer of brake grease. Now the wheel cylinders move with help from a good hit with a mallet, but I can't move them by hand. They definitely don't slide "freely".
How tight in the slot should they be?
Also, despite being an early model, mine had the late style wheel cylinders. The rear brakes never worked correctly since I had the car. From what I read online, it shouldn't be an issue but maybe there some weird fitting issue at play?