My Power Disk Brakes are frustrating me...

Ok, so I have a '74 Duster with power brakes, disks in the front and 10" drums in the back. The engine seems to only want to give me a max of 10"/hg of vacuum.

Anyway, so when I push the brakes, the the first push is good but the subsequent pushes I need to pump it. Regardless, I have to pretty much push the pedal to the floor to get the car to stop. Locking the brakes only seems like a fantasy at this point.

This is what I've done so far. I've replaced the rear wheel cylinders, I have flushed and bled the brake lines.

I could probably adjust the rear drums a bit tighter since I only adjusted it enough so that the drums can snugly slide on. Although, I don't think that will fix my problem.

At this point I don't know if my problem is any of the following:

1. Not enough vacuum to operate the power booster.
2. The booster is possibly bad
3. Calipers are bad

If the booster is good and the calipers are good, then should I switch to manual brakes or swap the cam? I do want a better idle actually and I do kinda like power brakes. Kinda got used to them in my modern rides.

I could post videos and pictures if needed.