Hard (power) brake pedal

Looks like the right master for drum drum.

Like to go with simple basic things first. Jack the car up on jack stands, have a partner step on the brakes while you are spinning the wheels by hand. See how much brake pedal pressure it takes to actually stop the wheels from turning.

Check the fronts and the rears. Generally the power master cylinders have the larger bores compared to the manual masters.

See what you can figure out. Did you put new rubber brake hoses on ? They can collapse internally being 50 years old and you can't see that from the outside. Makes for one stiff pedal when the brake fluid can't get through the rubber brake hose.

So here again, test spin and brake all 4 wheels to make sure the fluid is getting through.